Just an idea, not sure if it would work or not.
Does he have a good friend that he's willing to have come over and the 3 of you could sit down together and talk about how important it is that he does his meds, and ask this friend if he could keep him company while he does them. If he feels supported by even 1 of his peers, it might not be such a big fight. A friend who is responsible and will stay back or stay in a room with him while he does his TOBI. To keep him company so he doesn't feel left out?
Also, maybe talking to someone in charge about letting him go into the "adult" cabin or somewhere private to do the meds for 30 minutes so others don't have to see him, or if he feels comfortable with 1 (or maybe even 2 friends) staying with him in that private setting, then he won't be alone. .
Mark had a good friend, actually the are still best friends, that would tell him when he was younger, "we can't go unless you did your meds. We'll do that after you do your chest PT, I'll wait here while you do it. I'll stay with you while you do this medication". But most importantly his friend put his foot down and pretty much said, "we aren't going if you didn't do X". I'm not sure if Mark's mom ever mentioned to him how important the meds and TX was for mark. If she did, she probably mentioned it to him in private and asked him to not tell Mark they talked, but for some reason or another, he ALWAYS put his foot down about mark not doing his meds. I think that really helped him.
Good luck, this isn't going to be an easy battle regardless and it isn't going to be your last one either.