Brumation most likely. . . give hime a
bath and make sure he poops, if it all looks normal, don't "force" feed him and just leave hime be. As long as he doesn't look like he is losing weight and eyes are clear I wouldn't worry. The first time my went into it I thought I was in for a dead dragon. I just kept an eye on him and now he's full of energy!
Read the articles on this forum about it and see if it sounds like what's going on.
Nick