This will probably be an un popular answer, but I think for the sake of his mental health and his current and future attitude about sex, it might be best to minimize your reaction. I would not go to the police, but definitely would talk to the other parents and would talk to the school/camp about the lack of supervision.. I have two teenagers, and I would be very upset if this happened to either of them, too, but I feel like how you respond will make a powerful difference in how he processes this, for a long time to come. Though sexual play is normal for adolescents, it sounds like this was quite a bit more than simple flirting. These girls sound very aggressive, and they need to be talked to about their behavior, and they need to be taught some healthy boundaries. I just feel that responding too strongly might make things even worse for your son. You said that at first he was coerced but then began acting on his own- I imagine he has very mixed feelings about what happened and is very confused. Having to work that won't with a therapist (unless the already have an established relationship where your son feels safe) might be even more traumatic. He may just need some reassurance that all the feelings he has been having (negative and otherwise) are normal, and that his parents are addressing the situation with the girls' parents..
How is it going to negatively affect his mental health and attitude about sex to be told that NO means NO and that he does not deserve to be sexually assaulted?? I work for a sexual assault crisis centre and you would not believe how horribly negative of an affect it has on people - well into adulthood - to know that their assault was NOT taken seriously and that their parents didn't view it as serious enough to take the appropriate actions. I swear, I feel like I am in the twilight zone reading this responses. Again - would you say the same thing if it were a 13 year old girl and a 17 year old boy? I highly doubt it.