Here's where I'll throw down some fucked up science. You know how you get a hangover, right? It's not from drinking too much alcohol but your body craving it...well the same thing can happen with over-eating and being overweight. His body has gotten so used to it's size and calorie consumption that it takes a massive amount of food in order to keep his body functioning. He'll get withdrawal headaches from sugar, his temperament will fluctuate violently and chances are he'll binge the first chance he gets thus undoing any positive improvements.