The best thing you can do is hire a sports nutritionist. I'm a trainer and nutritionist and I've worked with many kids. If he can learn healthy eating habits so that he gets all of the nutrients he needs, plus learns how to eat right... he will be able to gain muscle.
It does take some discipline, but it can also be very simple. When I work with kids, I teach them about balancing protein foods, carb foods, and fats, along with veggies. Then I teach them how to eat snacks and keep their body balanced.
It will help his mood, focus, and body.
I hope this helps!
VP - PrepareMyKid.com