Your retaining a strength and conditioning coach is a very savvy move, very savvy. On that note - and please take this from a man who has been to a lot of ball games and have seen things that just didn't sit well with me.
Baseball can be a great time between father and son. Your pride shows through with every smile, I've seen this in so many moms and dads involved in T-Ball. T-Ball ... yes T-Ball.
Watch a T-ball game and think of all the crazy things kids do, just being kids. It's that time in their life when there's no worry about failing, not reaching their potential, not measuring up to mom and dad's check book on coaching, $150 bats, $200 gloves, playing fields that your National Guard once used as a bombing range, a lawn chair crowd that teaching kids how to insult umpires, curse like a trooper, and toss all kinds of crap on the field and walk away.
Your son should never get to the point where his work is.... work. His betterment must never be betterment, because it's paid for, and most importantly, he never disappoints you. Never.
Kids have a way of evolving and turning into something different than what others expect or think they see, or should see. During the growing years... around 14 to 16, boys are changing into young men, hormones are playing all kinds of tricks on the mind, body and sprit. So being all thumbs and feet, as it will, can make your son awkward at times and not in step with his training expectations.
By the way, one of the main reasons for a reasonable water intake, is to promote and compliment your son's digestive system. And in that process, your son's body will grow itself, at its own pace, and within your son's body tempo and management. Weight gaining stuff is a risky cocktail. Your son's internal organs are, and will, grow at their own pace, providing all kinds of secretions and breaking down foods at a certain pace, The liver produces bile that breaks down fats, and the pancreas produces insulin. Insulin is a very important hormone that helps to regulate blood glucose levels.
I've witnessed grown, mature men, invent cocktails for all kinds of so-called benefits, and it's a role of the dice for some.
Just remember, whatever your son decides to commit to, intake wise of foods and supplements, will have a long ranging impact on his health status later on.
A Register Dietitian is the smartest way to go if your going to be writing checks in this direction.