Welcome!  You're in the right place.  My advice boils down to one thing.  Search disengagement on this site.  Many people have shared your experience and you'll read about the why's, the how's and the whole theory.  Good luck.

You claim that your SS is not a bad person yet the way you’ve described him would indicate otherwise. He’s a liar, a user, a manipulator and a sneak with no motivation. His behaviour is hurting you and his family. What’s to like? Worse still, he’s driving a wedge between you and your DH (dear husband) with his lack of morality!

 You’ve stated that your SS and his mother ‘… are the only things my husband and I fight about.’

My own DH and I dated for 5 years, during which time we almost never argued. Yet, when we married and I moved in with him, disrupting his teen daughter’s status of ‘daddy’s only girl’, we began to fight. I had left my great job, my own cozy home, my family and friends to marry and felt absolutely trapped. Until my DH realized what was happening (my SD was generally civil when my husband was nearby; sneaky, like your SS), and became very supportive, my life was pure hell.

Hon, your home should be a place of comfort and peace, rather than a chaos of disruption. You are far too involved with this mess and need to take a giant step back. The lad is not your child and the less you entangle yourself in the relationship between SS and his father, the less stressed you will become. Fortunately, you only suffer the young man’s presence alternate weekends. My DH had 50% custody of his 2 teenagers and believe me, that was no fun at all!

The disengagement that JRI mentioned may be tricky at first but if you persevere, you’ll get the hang of it. Continue to be polite to your SS, greeting him when he arrives and saying ‘goodbye’ when he leaves but not much more. Do not engage him in conversation and do nothing for him; no rides, no financial contributions, no special meals, etc. If he asks for something, refer him to his father. 

You cannot possibly like, let alone love, your SS. What’s to like?! Ignore him! He’s your DH’s and his ex-wife’s problem to address and ‘staying in your lane’ was solid advice. Please give the ‘disengagement’ posts a good read. I truly believe that following the suggestions of other, fraught stepparents will make your life easier. 

Hello. I related to your post and wanted to reply. Like you, I have always thought of myself as a grown-up that had a positive influence on kids. I get along with all of my nephews, used to get down in the dirt and play with them. I led youth groups and took my responsibility seriously. I find nurturing the positive development of young people very rewarding.

I also relate to your story about how you got along with your SS, but your dh "warned" you about him. The exact same thing happened in ny case. SS was 13/14 yo at the time. He acted very sweetly, and I couldn't understand what SS meant that I hadn't seen his "true" self. I was not at all used to a kid/teen that age that was capable of such duplicity. SS would act apologetic and sweet, but I learned that his typical MO was actually to be angry and defiant. The duplicity really struck me in someone so young.

It got really bad between the ages of 14/15 until 17/18 yo. He became super vindictive, angry and disrespectful to me once he realized that I was going to take some attention from his dad away from him. SS is 18 now, and is a lot more polite and compliant, but he still passive-aggressively shows me contempt. The whole time this was going on, I struggled and still struggle to do better, be helpful because "I'm the grown-up."

In any other case, I wouldn't continue to try and appease and help someone who has been so hurtful to me, and shows in many ways that he has the knives out for me. But that part of me who feels duty-bound to care for young people keeps me stuck. I would do anything to avoid feeling like I am abandoning and neglecting a young person. That sense also gaslights me to the extent that whenever I react with horror at something SS has done, it turns into gulit and disgust with myself for essentially calling out his bad actions and choices for what they are.

I don't know if I'm making sense, but I just wanted to put words to a very confusing experience of coping with a young person who defies normal expectations, and trying to treat them as you normally would. But at some point, like the other repliers have said, I disengaged. It's still a struggle because that induces its own kind of guilt, but it made the situation more tolerable. The way I pose it to dh is that 1. it will actually help the relationship with SS in the long run, and 2. when someone bullies you, you don't make them like or respect you more by trying to appease them or begging for their love.

You unfortunately know in your hearts of heart. There is nothing you can do. BM doesn't care. Your SO IDK what's he doing if anything,  for your health, you must disengage.  As in what happens will happen.  I can't even say you will be there because it will blow up in your face.  Get a saving account for a lawer if needed. 

You know from school. Thus all started when this child was young,  parents were together.   You have seen kids in the first grade you knew weren't right, [ the ones, the teachers are talking about trying to help them]  and 12 years later in high school. There the ones in all black every day.  I have see parents told to get your kid help.  They changed schools.  To get out of the system.  12 years latter kid is in all black clothing .  So sad to see  

Don't get into the "he is isn't a bad person he is just making bad choices" bullshit.  Is his behavior reasonable and in compliance with the standards of behavior and standards of performance required?

If not, bring the age appropriate misery escalating consequences.

Do not fight with the why of his crap, just address the crap instantly and with effective consequences.  Make sure it is clear that by choosing the behavior, he is choosing the consequences.

Lather, rinse, repeat.

Take care of you.

And welcome.

Welcome to the site! Here is a list of abbreviations:

https://www.steptalk.org/faq#a2

For years, I worked with kids and people of all ages with trauma background that were in foster/residential care/juvenile justice/adult prison system. I was always lauded for connecting with some of those hardest kids. I eventually distanced myself from that type of work, because after some years I was able to do my own informal longitudinal observations to conclude that despite everyone's best efforts and the insane amounts of money spent by the taxpayer and NGOs, the damage caused to them and the genetic loading always won.

I also saw many of those kind social workers, foster carers, residential workers who truly loved those kids suffering the most awful repayment: endless exploitation then discard, family heirlooms stolen and destroyed, beloved pets hurt and dead, marriages destroyed, all manner of stress related illnesses - heart attacks, coma, cancers etc. 

Yet, those very kids were very sweet and able to connect with me when I saw them. What of it? There was no lasting and no knock-on effect. I saw most of those kids eventually follow exactly the path of their criminal, unemployed, good-for-nothing parents they had been removed from, even though some were removed at birth and had very limited, supervised contact with birth family. 
 

When I met my boyfriend (a widower, kids with him full time), I thought I'd have no issues connecting with his two kids who seemed sweet, shy, if a bit hyperactive. Further down the line, I learn their maternal immediate family is ridden with severe genetic disorders, where EVERY SINGLE person has a physical or mental disability, or both. That included their mother. With adolescence, some of those came out, we got them diagnosed and medicated, but I can see that there's more to come, and that the kids are becoming a spitting image of their maternal family members, in behaviour and appearance. Thanks to this forum, I kept my home and did not jump into marriage. They moved closer to me and we see each other many times a week. This suits my very well. And I disengaged big time. It's not easy, because I see exactly what the problem is and at least how to begin fixing, and I can relapse a little from time to time, but I then think of all the kids that I worked with where ultimately they jumped into the very muck they came from, and it's a lot easier to prioritise my sanity.

The bottom line is that you cannot care more than the bio parents.  While I understand that your background "should" mean that your input would be valued and welcome.. the reality is that the term "stepmom'dad" is kind of a misnomer.  You aren't his parent. He has two that should be raising him appropriately.  And, yeah.. that does sound a little bit like "stay in your lane".. because the lane you stay out of also is the one that has the semi trucks that can run you over..lol.  It's not that you are butting out because your advice wouldn't be good.. but because it is often not welcome.  

At his age, the die is somewhat cast for the kind of person he is.  His parents raisded him to be how he is.. along with their genetic tendencies.. he may mature somewhat.. but in some ways.. he is what he is at 17.. and there is not a lot that can be done to change the personality or person he is growing into.

Unfortunately, for stepparents stepping in "midstream" of the child's life.. any attempt at correcting an errant course feels like a person that is an outsider is trying to tell people what to do.  Changes that may well be "good" for the kids may not be welcomed by the kids.  for example.. setting better screen limit times.. bedtimes. or chore responsibilities on kids who never had them will be resented.. and "you" will be the one to blame for their loss of "freedoms" and for "more work".

If you catch it young enough.. with enough all in support by your partner.. it is a bit easier.. but the older the kids are.. the more they are going to resent another authority figure in their llives.

And.. as SP's we have all stuck our neck out with these kids.. just to get it chopped off.. for things to blow up in our face.

So, what I suggest is that we let our partners parent their own kids.. we can have expectations with the partner about how they allow the kids to treat the home and residents of the home.. we can set expectations of whether the kids will live at home after HS.. etc.. but in the end.. the parents need to be the ones to truly parent.. as SP's we can play supporting roles.. help with rides.. or fixing meals.. doing laundry etc.. we can be friendly and civil with the kids.. we can be like the fun and wise aunt... give advice when ASKED.. and our partners should respect our opinions.. but in the end.. it's hard when there is another household that may have some different ideas.. and can't be controled.. so we have to sometimes just not invest our hearts in any particular outcomes.