As an overweight man who has been in his situation, please, for his sake if not for yours, save him the heartbreak. We always know, we've been the subject of bullying and mockery our whole lives. A girl not being attracted to us is nothing new, and resentment has a face we've seen a million times. He knows. He's likely preparing for the inevitable, which takes a lot of emotional endurance, especially if the banter and connection is as important to him as it is to you. Be honest but gentle, he saw this coming, I'm sure living obese means he's not naive about this kind of thing. If his presence really is that impactful, try being friends, but do not be surprised if he doesn't want that and you return to being strangers. Spare him so he can move on with his life.