The majority of teens have no reason — medical or otherwise — to try to gain weight. An effort like this will at best simply not work and at worst increases body fat, and may put your son at risk for health problems.

He should instead focus on growing strong ... not gaining weight. Keeping his body healthy and fit so that it grows well is what's most important. I did find some website which might help you .... check them out!

http://kidshealth.org/teen/nutrition/weigh t/gain_weight.html
http://kidshealth.org/teen/food_fitness/di eting/weight_height.html