A big appetite or what?

I do lawn and forestry work in my business among other things. I have 2 crews of 3 men. As part of their salary I buy lunch daily up to 6 bucks each. I have this 1 guy who's 31 years old and a good dependable man. He can eat more than any human I've ever seen. If we go to Burger King he gets 3 dbl wpers and 2 giant fries. If we go to Krystal he gets a 24 steamer pack. It's like that everywhere we go and he never gains an ounce nor slows down after eating. Everybody else watches in amazement. I have to eat salads and drink water because I'm diabetic. But how can this guy eat like this and never gain weight? He's always hungry. I give him the 6 bucks and he has to pay the rest. His food bill daily is over 20 bucks.