Play to his fantasies. (No not those ones…) Every guy that age has something they wish they can be when they grow up. Those dreams change as time goes on, but in the meantime it’ll help build confidence, give him a “cool factor”, and a chance to dream.
My memory of the pudgy kids that lost weight because of these dreams:
A guitar or other “rock star” instrument. Lessons help, but he’ll get bored or frustrated with the wrong teacher, so be prepared to switch teachers if he wants to give up. Lots of room to dance around with the guitar, and those pounds sweat off pretty quick. Buy good earplugs. Actually, you don’t need to, an electric guitar with a cheap “personal amp” plugs in and plays through headphones, and works plenty well enough for a beginner.
Skateboard: If you live in a skateboard-friendly area (lots of pavement, lenient cops), just a couple tricks might be enough to keep him going, especially if he has a decent board. Buy pads, “cool” skaters don’t wear them, but the best ones don’t skate without them. Takes the fear out of falling. Similar idea with roller blades/BMX bikes. If you have a decent sized yard and he takes to it, building a half-pipe is sure to win friends, too. Free skate parks nearby help, too.
Whatever sport he likes, if it’s available. Check his video games/tv/magazine favorites, there’s probably something there. Something where weight is an advantage for younger beginners, especially if he’s physically strong.
Weightlifting: At his age you don’t want too much weight, he probably can’t lift too much anyway, but a bare bar and a couple light dumbbells can play to the Conan ego. Repetition is the key at this age. A little extra muscle can burn calories day and night.
You get the idea, individual “sports” to start with, allowing him a chance to work at something with a friend or mentor, until he’s confident enough to run with it on his own, join a group/team, or morph it into something else. Maybe watch the guys a little older than him, figure out what they like to do, and put something together. He’s probably a little jealous of someone, feed that jealousy…
Fat camp might mess with his confidence, unless he’s eager for it, and any forced sports, especially with large teams can be a little traumatic. At the same time, he’s probably going to resist anything you offer, but push a little and see if he gives in, maybe offer him a deal, try it for a short length of time, and if it’s not fun, next idea. Remember how slow time passes as a kid though, months are forever. A couple weeks, a handful of lessons, etc., should be enough to gauge his interest. Just make sure it’s his idea to continue, not your own missed childhood dreams.
Diets at this age aren’t going to go over too well, he needs to change his activity, restricting calories runs the risk of reducing nutrition, and he’s at exactly the wrong age for that. Even a healthy workload of chores might be someplace to start. But playing to his dreams is probably the best way to go, and might make you a pretty cool parent, which is something valuable in itself.
Mostly, don’t make it about the weight. Nobody likes diet and exercise, but we all have things we wish our parents would have let us try our hand at as kids. So does he. Good luck!