i agree with ickydog. if he needs to gain weight but has difficulty eating, he can try liquid calories like ensure, boost, milkshakes, and fruit smoothies. i would make sure they actually contain good nutrition though, not just fat or calories. if he doesn't get enough nutrients he could still have trouble maintaining weight, even if he's getting enough calories. if he's more of a "snacker" than a full meal eater, you can also keep protein and meal bars around. they are both calorie and nutrient dense, so you get a lot in a smaller package.