Who is paying for the trip? I would not be all that willing to pay even my portion of a trip where my preferences were not considered and I was having a lot of anxiety because I couldn't figure out the expectations for the trip. Your desire to get an understanding of what this trip will be is completely reasonable. Do you think your boyfriend's failure to tell you what to expect is because he doesn't know himself?

I haven't gone on a vacation with teens, but I can tell you with certainty that any trip we took with my DH's kids (including those before we were married) was not relaxing in any way and was all about the kids. 

It may be tolerable if you and your husband have a separate bedroom and bathroom from the teens and if your husband stops being an a-hole. It's good the son is bringing a friend. That takes a lot of pressure off  the adults. As long as your husband and stepson aren't major jerks (and i have my doubts based on how he's already not listening to you) it could be fun.

ETA depends a lot on the trip, though. Some places allow for teens to go off and do their own thing. The separate sleeping and bathing areas are also a must. 

Your BF is being selfish not giving you a say or doing things you want to do. With that said, I've done a few trips with DHs kids. Add in I have my own kids so I'm aware of how it is to travel with kids at all stages. Every trip we took with DHs kids came with massive drama. After about 3 times, I was done and I've never traveled with them again. From the Passive Aggressive BS to out right blow up arguments do taint and / or ruin vacations. 

You don't say the dynamics between you and the kid, or if your DH has turned into a Disneyland dad but based on his treatment of you so far on this trip....I'd have to re-consider. And especially if you are contributing to this trip, no way would someone tell me I can't do something I want. 

every right to ask.  You not only have the right to ask, you can stipulate.

So, ask. If the answer is unacceptable... then stipulate.

smh

Been there with TWO SDs. Will reply tomorrow. You should be able to scroll through my blogs to read about my experience. Most depends on the skid relationship with the bio parent and you in daily life ...

So you suggest one thing you want to do and SO won't do it? It sounds like your real problem is going on vacation with your SO, not going with his son. What kind of a vacation is this going to be for you if you have to do everything your SO and two teenage boys want to do? Doesn't sound like much of a vacation. Does he always demand things go only his way?

Was his reaction to your questions typical of his behaviour? Does he often disregard your opinion? 

If it is, I would decline to participate in the trip. Tell him what a great opportunity it will be for him to get to know his son better and to bond with him.

If you do decide to go,stick to your guns about only paying for your share AND insist on the one thing you wanted to do be accommodated AND on being told all the details of the trip. Sharing a room with his son and friend would, of course, be a deal-breaker.

Trips with teens end up being all about the teens or at least highly impacted by them. Unless you are dropping the teens off at a camp or something then maybe you'll get some time as a couple? 
I traveled out of country with La Diabla when she was around your SS age. She monopolized every moment she could with Daddeeeee even throwing a fit that daddeee wouldn't sleep in bed with her. Fortunately we dropped her off at camp for half the trip so I got a break from the little bratzilla. 
It is a rare 15 y/o that is tolerable. That is a hard age even if the kid is your bio.

One time DH took SD's overseas, I opted out and stayed home. Was it annoying that he took brats for such an expensive trip? Yes. But it is better than having to spend my vacation with brats. I went to visit family and had a great time by myself while they were away. Now that they are young adults the free trips have finally ended. Thank god. 

I wouldn't go if I were you. Let them have some "alone" time. 

First you must have separate rooms.  I would not go on a trip with out my own room.  
I think it's fair that you get to get to decide what to do for atleast one day. 
Thaveling With kids. You normally try to do things kid oriented, amusement parks,  hip restaurants, fun activities. To give the kids a childhood memory,  keep them happy, normally BP get pleasure from haveing there kids happy, unfortunately this does not apply to SP.

I haven't travelled with teens but definitely with skids and my DH attention was definitely on them a lot. It was frustrating at times we couldn't have time to ourselves. If they wanted to head somewhere and I didn't feel like it, I was left on my own alot. This trip has red flags already. The fact your bf isn't listening to start with isn't a good sign at all. 

Ok, finally I can respond.

I've been on local and trips to other states with the SDs. Trips for fun and trips to visit family. We've brought both of them overseas. Here's my advice, especially since you have never travelled with his kid before (!!)...even locally?

* Are you going somewhere YOU want to go as well?  If your BF unilaterally decided this trip you do not have to go!  Is the trip for he and his son?  It sounds like it if he's not letting you do things YOU want to do. When DH unilaterally decided to take SDs (then YSD13 and OSD15) on a 10 day trip with us to visit his family overseas, and after many arguments he would not back down I made it very very clear that there were "x" things I wanted to do no matter what and the SDs and he would not stop me. I wanted to go because I love his family and the area they live. I have chosen before to NOT go on trips with them. That is ok too.

* Are you paying for ANY of your BF/son's costs?  If so, NO. You cover your own costs. Your BF must pay for himself and his son. This includes entertainment and excursions...maybe not food, but yes, the majority of food too. I made it clear to DH that because I didn't agee to the SDs coming that I wouldn't pay for them (I ended up helping with some of YSDs costs because she was OK and ended up being a good traveller). I would help pay only to do things I offered to do or wanted to do with everyone. Otherwise it was on him.

* Are you going somewhere where you all can speak the language? How is his son with unusual situations and handling change/lack of sleep? Is he patient/impatient? How is he around strangers and crowds?  If he's not naturally social and inquisitive it will be more difficult in airports and traveling. Both SDs were overwhelmed and sometimes rude when traveling. The clingy behavior was at another level OR suddenly OSD would immediately want to leave somewhere and argue incessantly even if DH and I were not done. OSD had no curiosity about where she was and constantly complained. Neither SD bathed for almost a week until DH's siste said something to him.

* How is the kid with food and different kinds of food and eating in restauants/cafes/on the go with little food options? OMG - this was awful for us and I predicted it. OSD and YSD and extremely picky. I told DH that on top of him being the one to corral and decide on things/keep SDs in check (I would only step in if it was over-the-top needed), HE was the food person - HE had to decide on food options and feel the pain. And he did!  Oh he did.

 * Do not share a room with them unless it's necessary. DH found rentals with 2 bedrooms and he paid for an extra hotel room for the SDs (across the hall from us) when needed. You will need space and sleep.  I also demanded 2 days alone for DH and I - doable because the SDs stayed with family. We got about 1.5 days and one night away at a lovely place with no teens. This will be almost impossible for you if you have no help. If the son and friend cannot be trusted in a hotel room by themselves that is also a sign they are not ready for this trip maturity wise.

Basically you have no responsibility for the kid. It's all on your BF. I would hesitate to go if I were you since you haven't travelled with him before. You need to take into account what kind of kid/teen he is. Does his son WANT to go? Is he exited? Is your BF involving him in the planning?  If you get to do nothing then do not go. In fact, I would be reconsidering my whole relationship. I would never tell my DH on a trip that he couldn't do something he wanted to!  We don't dictate.

With his spawn and the spawn+1.

Keep it simple.

If SO gets cranky about it, tell him that next time he should speak with you and listen to you to make sure the agenda is set or he will not have any time with you on the next holiday either.