Sending good vibes your way, as well as kiddos way. 

I would try and push them to wait until closer to 25 for surgery. That is when the brain is fully developed and they will have a better sense of self. Maybe they will be ok with waiting a little while and see how the hormones work. I'm not sure how insurance coverage works for things like this but I don't think it's something covered by most. Don't financially support them either in this effort, at least for now. 18 is too young to make this decision. I would rather them get married at 18 than have these surgeries, at least there is divorce and you can move on. Removal of breasts is forever. Even if the decide to get implants later, that surgery is so extreme. The take tissue from your stomach or thighs to recreate the breast. It looks like Frankenstein. There is no sensation, no nipple. Just try to convince them to wait for a few years to decide 100% on the permanent things. 

Hormones, sure, go for it. Clothes, binders, anything cosmetic- fully support! Go to counseling together. Find someone who isn't pushing an agenda but genuinely wants to help them be ok. No matter what that looks like on the outside, they need to be good in their heart and head. 

I'm "old".. and while there certainly was a good amount of popular lbgtq type influences in media when I was a teen (Bowie, Boy George, George Michael...etc... ).. I would say that local to us.. it wasn't as big of a thing to see many people in real life going in that direction.. being a military family .. our universe was probably a bit smaller too.  Since Graduation, a few classmates have come out.. but it certainly wasn't as much of a "thing" as it is now.. and whether it was because of worry at being accepted.. or whether it was because kids didn't really think of it as an option.. and if they had leanings didn't see that as a possibility.

I will preface this to say that I do feel that people are, to a certain extent, born with some inclination and wiring in their brains towards being more masculine or feminine.. and to attractions.  I do think that there are a lot of kids out there that are searching out or looking for something in their lives because they are unhappy.. dealing with mental health struggles.. or even are looking to rebellion.  I'm not trying to take away from people that are truly dealing with gender dysmorphia.(sp?)... but sometimes I think it's kids who just don't know who they are.. or where they fit in .. teen angst.. that leads them this direction.

I was a very, very masculine leaning child.. refused dresses.. refused any girly direction.. at 5 quit ballet the first day of class.. got kicked out of the brownies.. liked boy stuff.. etc..   But... when I was probably 14.. I did start to gravitate to more fem things.. though I still enjoy drinking a cold one with my DH and his friends.. don't really enjoy shopping.. like casting a line and catching fish.. more than my dH does..haha.. and while I do have some more fem qualities.. am still kind of tomboyis in many ways...

So, I wonder if some of it is people rejecting a club before or because it rejects them?  Teens with little dating success.. or interest.. feel something's wrong with them.. they must be not "typical".. etc.. and it certainly is much more accepted and in some circles even encouraged to explore these possibiilities.

I guess in short.. I do think it's real for some people.. but probably not for the amount of kids that try on that persona when they are young.. But.. being dismissive of it to your child wouldn't be fruitful either.

I think the big thing to do is to be as educated as possible about the risks of transitioning meds.. and the risks of unneccessary (medically) surgery.  I would encourage them to put off medical intervention as long as possible.. and to insist (as far as you can insist with an adult).. that they seek out some legit therapy to help them work through these ideas... to make sure it's a reality vs an attempt to escape or deal with other stressors.

dressing.. presenting as a different gender is fine.. be as masculine as they want to be.. but hold off on meds until they can truly weigh the risks.

That's excellent that you want to be supportive. 
 

Just opionion only but I do think being older than 18 like at least 25 would be better for irreversible changes. 18 is still so young and "figuring things out" stage imo
 

But other than that I'm sure your kiddo will appreciate the efforts you make to be as supportive as you can

My Dh's sister's kid is trans.  They were born female.  Started dating a guy in high school.  This guy came out as gay.

Niece came out as trans and started taking hormones.  Their voice changed and they grew facial hair.

Trans nephew and boyfriend eventually got married.

Fast forward to now......they decided they wanted a baby.  Went off hormones and had multiple miscarriages.

They are now pregnant and due in January.

I guess they had to stop the hormones and will go back on them after the baby is born.

I guess if they are going to breast feed, it's a good thing they didn't have surgery right away.

I know quite a few happy, successful trans people. The expense to transition is a real concern, but do try to be supportive. Seek out local LGBTQIA+ organizations for guidance - they can help!

Have they started seeing a therapist who can help them deal with transitioning?  Have they started looking to support groups of other trans people who have transitioned or are in the process?  Have they begun researching medically what this will cost THEM (not you), and what they can expect from taking the hormones and from removing their breast tissue?  

When my BD17 decided she wanted to be trans at 14 I made her do her research of what she was going to be getting herself into, I also told her we needed to find a therapist who had knowledge in transgender people and who knew how to help them process all of this.  It didn’t take long for BD17 to decide she really wasn’t trans.  I am not insinuating in anyway your child is not, but they need to start doing their research and understanding what this will cost and how it will affect them mentally and emotionally.  

If you have never watched the show “I am Jazz” maybe watch it.  For me it was a cautionary tale of what happens when you allow a child to make such a monumental decision about their body at such a young age.  Jazz to this day still struggles mentally and physically despite claiming she would just be happy once she finally got her bottom surgery.

I think your child understanding what they need out of life is great, but at 17 they don’t always understand how long life really can be.  I hope with your support they can begin processing what all this entails and understand the effects it will have on them.  Wishing all of you the best of luck and safe journey on this path.

My nephew is trans.  Always tomboyish and stated she was trans at the age of 16.  Did wait until 25 to start the transition.  This was all with full and loving support of our entire family and friends.  It took a little over 2 years with a lot of therapy.  A Lot.  

We all went to therapy groups together.  First because we had sooo many questions that some of us felt uncomfortable asking without insulting them and 2nd so that he could hear our concerns of his future, marriage, children, etc.

He is a successful banker, married with a 4 year old boy they adopted and just had a baby girl 3 months ago from donated sperm. 

He did teach me that, yes it was hard on his parents and some of the family members, but it was so much harder on him.  They question themselves every second of the day on everything.  Should i or shouldn't I, why can't I be what everyone wants me to be, will they still love me if I do change, etc.  

I don't want to give the wrong advice because full disclosure, I have zero experience in this but... I would definitely agree on the waiting a while, experiencing the world beyond the small town, etc because the more people they meet, from all different walks of life, maybe gain some new friends, happiness increases... then I bet would be in a better head space to make this decision and maybe it wouldn't feel so much like this is something that NEEDS to happen asap. 

College is great for this, there are groups of like-minded individuals who can offer advice, peer support, etc all while they pursue their education. 

And two step daughters I have seen so so many girls wanting to transition to being males. I think there is a reason for it . I think they see how hard it is to be a woman and it freaks them out to be ogled and sexualized. I would say every single one of my kids friends transition back but then again they were all feminized children. My sister's stepson is transitioning to be a woman and she is 19. The surgery and hormones would make me extremely wary and nervous as well. I think it's very easy for me at least to spot in my children's friends who is really trans and who is just jumping on the trendy bandwagon. They change their pronouns and genders and names like they change their underwear. I still abide by their wishes and use whatever pronoun and name even if it's a moon person made up gender and sometimes it is. It's hard to take some of these kids seriously but you never know when a real trans kid is watching and hoping for acceptance and a safe space and ally. It irritates me when they use the trans movement just to try and get attention because I think it makes a joke of the struggle that real trans people experience and diminishes their cause but I try and remember that these are teens trying to find themselves.  You know your child best. I personally would take him more seriously since you saw many masculine traits as a child but I agree it's definitely not without risks. My coworker lost his trans daughter to cancer in her 30's and I do believe the cancer was a result of the hormone therapy. Giant hugs for you. It's hard 

parent is a delicate one.

I would let them know that they are on their own with the transition surgeries.  That you love them, support how they identify but that you cannot and will not be supportive of them hacking pieces of their body off.

If they choose to pay for it themselves once they not only reach the age of majority but are fully self supportive, then that is their choice.

My condolences on your child's severe case of body dismorphia. 

Though not the same level of challenge either as a parent of for the kid, our son came out to his mom and I as a gay man when he was 22.   He has struggled to reset and grow up as a gay man.  He came to the realization when he was 18.  He dated young women in HS, had both male and female friends, and was for the most part a reasonably happy and well adjusted young adult.

Once he realized he was gay, he had a few rough years as he worked through the realization, the guilt feelings that something was wrong with him, and reconciling the whole process for himself. 

As his dad, I was relieved and not at all surprised. I had suspected for a number of years and was confident of it when he was about 17.  I "knew" before he did.  His mom was blind sided.  We both wish he had come to this realization earlier so we could be there for him.   When he came out I told him that he was our son, we loved him and that this did not change a thing for us as a family.  I did make one request of him. Keep his head in the game, be fully aware of his surroundings at all times, and protect himself because not everyone would be accepting of it.  Life could be hard enough and this might make it more difficult for him.  

After a brief pause he said. "Dad, you know me. I do not like attention.  I am not going to put on leather chaps and a tutu with my butt hanging out and march in some parade."  All three of us laughed through tears at that.  Not tears of hurt, tears of love. He is an outstanding man of character, honor, and standing in his profession and community.  His mom and I are very proud of the man who is our son.  He is a masculine beautiful man.  Male  fitness model beautiful.  A chisled jaw line, tall slender and muscular, with a perfect beard that he can grow by lunchtime after shaving in the morning. I am jelouse of his beard. Mine looks like an albino Chia-pet with mange if I try to grow one.  He gets the beauty from his mom (though not the beard). He is my former SS who asked me to adopt him when he was 22. We made that happen.

If he had dropped the actively transitioning trans realization on us, I believe we would have been supportive of his new identification though not suportive of him wrecking his body and health.