I don't think he has a big stomach for the reasons you think he does. He was asked about this. He stated that his belly is not going anywhere.
I think its obvious with the amount of weight training he does, and if you look at his back, which is actually half decent, there is a very specific reason why he has a distended gut. I just don't think its for the same reasons that a genuinely overweight person has a problem (eating too much). I suspect its medically-related.
I think his abdomen is permanently distended caused by many years of being very fat, with the gut hanging down, out of his shirt.
Saw a vid of him powerlifting back in the day, the stomach was just hanging way down below his waist line.
Permanently disfigured because of that (my guess). Constant weight of the gut pulling down the abdominal cavity...
He's pretty funny on t hose vids though, sick bastard...