Speaking as a kid who was turned off exercise in middle school because PE was socially traumatic and the coach was a festering asshole . . .
1.) Get rid of the TV and the computer or drastically limit the time he can spend in front of them. No more than an hour a day.
2.) Get rid of the soda, fruit juices, and sports drinks. Stick to milk and water.
3.) Indulge him in whatever healthy physical activities he’s interested in.
It took me until my early 30s to be comfortable doing physical things because I’d been teased and mocked so much during my childhood. The things that got me active again were the things I loved doing as a kid or was interested in as an adult - bicycling, yoga, swimming, racketball, gardening, indoor rock climbing, scuba, archery, fencing, target shooting, and other stuff. A lot of that list are things I’ve only tried once and would love to get back to, time and money allowing.
If you have the financial resources, get him a bicycle. Get him a Wii. Get him something cool and active that he will enjoy. And if at all possible, join in with him. If he’s the only person doing whatever, he’ll know it’s about his weight and not about living a healthier life.
It’s late in the year for this, but next summer, consider getting him a summer pass to the nearest water park, and find a way for him to get out there at least once a week. Send him with his own cooler of fruit and healthy snacks, and let him run around to his heart’s content.
If you look at Weight Watchers, find a men’s group - a lot of the branches now have meetings primarily for men. It would be so much more comforting for him to hang out with a bunch of guys than his mom and a bunch of women.
Please, please, please, don’t send him to a “fattie” camp. Send him to an active camp that stresses a healthy diet. They’re out there. Find something that interests him with a schedule and stresses healthy diet.
Good luck.

But if I did…well, multiple intelligences and all, I’m sure he/she would be…I would work tirelessly to find a loophole! :smack: