Bad carbs, low fat diet and constant eating 6 or more times per day in the form of sodas, latte’s and other snacks without giving the digestive system a rest is probably the biggest reason many of us are fat. I think genes predispose people to storing more or less fat too but the carbs are the biggie.
Now, to put this in context, this means that he would gain a pound every three days (you don't gain a pound of fat when you gain weight, you gain muscle and connective tissue as well so let's forget the 3000 calories equal a pound gained nonsense) if he were eating the recommended number of calories. In a year this would represent a gain of about one hundred and twenty pounds. So in addition to genetics, and diet, and exercise (a minor factor; you can't exercise enough to make a big difference and it stimulates appetite), there is also the body's homeostasis and its drive to preserve the status quo. Earlier studies of bushmen and others found similar surprises; given their extreme activity level you would expect them to require a very large number of calories per day to maintain weight, but they maintained weight on about half of that. Anyway, point being it is not just eating too much. And no, I'm not overweight.