I could be way off the mark here but I'll jump in with this:
He may well be depressed, even if he doesn't outwardly appear so. He may be very self-conscious, anxious or feel intimidated about going to school and interacting with other people. This can happen to people, even if they were once very social, especially in the mid-to-late teens.
Gaming could simply be a comfort, he can stay entertained without needing the socialize outside of his protective bubble. The stealing is also a warning sign, it's possibly reached the stage where his values and sense of morals have been pushed to one side completely as total apathy and inertia set in. Coming down too harshly could provoke rage, stubbornness and reckless behavior, but it's actually quite important that the issue is addressed sooner rather than later, waiting for him simply to get better isn't enough. I've seen people in his situation go on like that for years and years.
Does he react aggressively if someone suggests he's played too much, or that he might be addicted? He may even have admitted to himself he has a problem, but is ashamed to admit it to someone else.
As regards to fixing the dilemma, your family are going to need to lay down some rules, bring some structure back into his life and step in with some authority. He's being allowed to continue to ostracize himself, ditch school, continue stealing and break your family trust without any severe repercussions. He may well be using his illness as a shield and justification for his conduct, but he needs to learn to live without using it as an excuse for his bad behavior. A complete change of environment might be what he needs, but perhaps most importantly, he needs to talk and tackle some of the issues that are making him retreat into his own world, even if he doesn't want to.
