ReallyTired I am sorry you son is having these problems.
If he wants to go to the camp, and you can afford it, great... but if he does not want to go, how will you get him there!
You said I believe he is depressed will this be addressed at the camp, will he be helped? Because if not, I am not sure how the camp will really help him.
The friends he makes and contacts will be linked to the camp, will there be any on-going support afterwards?
If you are keen to do this I think it would be good to ensure your son is on board. You could also try a trial run with a personal trainer, someone really nice and approachable, not someone who will humiliate or overload your son.
Can you encourage your son with sports or activities that may motivate him. I took up Taekwan-do a while ago and love it.
Good luck, I hope it works, being over weight is not great (I know, I am) but it is also quite complex and really not something that can always be 'fixed' by people outside the overweight individual themselves.
I am sure you know his BMI, is it very high? I am not sure how the 99% relates to this.
Hope your son will be fine.