Males can stay surprisingly healthy at very lean body compositions. I wouldn't worry just yet. If he's hungry, he'll eat. If you just want to be sneaky and make him hungry and gain weight, lower his protein intake and he'll likely make up for it by eating more fat and or carbs. But that's a mean trick, and health effects are poorly understood. When you can bounce a quarter off the connective tissue at the base of his 8-pack, and count the striations in his deltoids unflexed, then it might be time to mention his body fat being too low.