Thomas has muscular dystrophy. This means he's going to gradually lose strength and control in his muscles over time. He has already lost his ability to stand, and even holding buttons on a game-pad for extended times can be difficult.
Gaming, as you can imagine, is very important to Thomas and people like him. It offers a release of frustration, like it does for all of us, but also a level playing ground. When he's in the game, he's like everyone else.