Hey, I'm in more or less the same situation as you. I'm 22 as well and my boyfriend is going away in August with all of his friends.
He's been away before with his friends and I've been fine with it, but for some reason this year I am a bit nervous about it!!
A lot of his friends have GFs too, but a few of them cheat on them. And I know that all they are interested in when they go out is talking to girls!!
I'm counting on the fact that he's going to be so out of it on the drink that he won't be able to talk to girls! lol!!
I don't actually think he would do anything and i do trust him, but it's the not knowing what he may get up to that annoys me! How i'd love to be a fly on the wall on that holiday!!
If i were in your situation though, I couldnt help but be a little worried about his ex being in the same place - although if she lives locally to him now there isn't a lot of difference in the chances of him seeing her.
Don't get yourself too down about it though, I'm sure we're both worrying about nothing! Your boyfriend is with you now for a reason, not his ex, so just think about that. And imagine how good it will be between the two of you once you see each other again after those 3 weeks!! ;)
Hope I helped?!