One of my brothers was honestly like that growing up: he could eat huge servings of anything and never gain a pound. He was always the skinniest in our family even though his after-school "snack" was two full bags of microwave popcorn, two pizza pockets, and three cans of pop. Then he'd ask my mom, "When's supper? I'm starving." Then he'd have at least two heaping helpings of supper. And then some more junk food before bed. He also was not at all physically active. He'd watch TV and play videogames, and his extracurricular was the drama club.
I used to think it was just not fair, but now I know it's because he has a much faster metabolism than I did. It's still sucky that he got a wickedly fast metabolism and I got one the speed of a sloth dragging a two-ton brick, but that's the way it is, and I have to learn to deal with the metabolism I have, not the one I wish I had.
Also, now that he's 30, it's catching up to him and he's got a beer belly. Ha!