How to gain weight for fast metabolism

My son is 15 years old. He is 6’4”, but only weighs 157 pounds. His exit velocity is 88.1mph. He works very hard on mechanics as well as conditioning. He is religious about arm care as well. My question is how to get him to gain weight to up his velocity.

Lift weights and eat like it’s his job, that is the simple version.

Lift- Get on a program that is designed for overhand athletes and work it hard. My 15 year old is in the weight room 6 days a week right now, he has gained close to 15 pounds since August.

Eat- Getting enough calories, and the right mix of nutrients, can be hard…a kid can only take in so much volume. The key is eating dense foods. Get the right amount of protein at the right times of day. Do some research online for the amount of protein and ideas for how to get it. My son has a second lunch box in his backpack and is fortunate that his teachers let him eat in class. He has a snack or small meal almost every class period.