Is it normal to gain no weight at all?

I never really paid attention before because my boyfriend and I lived apart, but now that we are living together I have noticed that he eats non-stop and gains absolutely no weight...he is in his early 20's and weighs roughly 150 lbs. When I say he eats non-stop, i literally mean non-stop. For example, one night he ate a whole medium pizza, in about 20 minutes. After he finished the pizza he ate a whole pie plate-sized dish of chicken and biscuits. After that he ate the rest of the dish of green bean casserole. He eats like this all throughout the day, everyday. The thing I am worried about though is he gains absolutely no weight. He does no physical activity, he sleeps ALOT and sits around for the most part. The only time he really moves is when he goes to the bar. So my question is, is this normal for him to eat like this and his body weight be as it is? Or, may he have a medical problem?