All my nephew does is play video games and he weighs 200 pounds in the eighth grade, with no social skills or ambition that I can detect. Which is cause and which is effect? I can't really say. I just find this hard to believe.
Buy him a DDR game. He'll lose weight.
Seriously though, it's not playing the games that makes them gain weight, it's what they eat.
My son is 6'1" and only weighs 140, wish we could put some weight on him. His dad has him working out on gym equipment and he seems to be gaining muscle, but he's still skinny. But he doesn't have an appetite for snacks.
Eats his meals and that's it.
My buddy's little brother was like that. Fat little video game player. We thought he'd never get a girlfriend, but eventually, after he became a hairy stoner, he hooked up. And meanwhile he took his tinkering skills to college, got some tech skills and got himself a job at Sony. Made a pile of money, decided he wanted to be a pilot, and now he's flying planes down in south jersey. you never know how they'll turn out. Oh, by the way, my old buddy, his older brother? He's an out of work divorced disc jockey. go figuree.
I turned to video games because I was bad at sports and didn't have much in common with other kids my age. The real world wasn't working for me, so I preferred to interact with games where I could understand the rules and get positive feedback in the form of game success. When I got older, I adjusted socially and found my niche.
This may be what's going on with your son.