Son isolates himself, won't do physical activities?

Hello all, I'm hoping someone can provide insight into this situation. My step-son of 3 years is 13 years old. He has a few friends and is social "sometimes". He is very social on the internet and while playing xbox live, but otherwise, not so much. He almost refuses to leave his room. We try to get him to do things with the family like watch movies, or just come out and chat. He says he doesn't want to almost always. He won't try to do anything physical. When his friends come over, they want to take a few swings at my punching bag. He immediately says "I'm going inside" and quickly walks back inside so he doesn't have to be part of it. He won't run, jump, play, ride his bike, nothing. Things he used to do regularly. I know it's because of the internet and games.... Here's the issue at hand, he was accidentally placed in strength training at school instead of PE because of a school error. He literally had a meltdown and started crying. He has had to attend several strength sessions and when asked about them, he has to choke back the tears. His friends say no one is teasing him or anything. We've learned that he may have to remain in this cl due to over-scheduling of other cles. What do we do? Why is he so adamant about NOT doing anything physical. He won't talk about why, he just doesn't want to. Advice?