C2019,
Good stuff. Just a heads-up, he will lose anywhere from 10-20 lbs of weight from beginning of HS ball to end of summer travel season. Just a fact that he cannot keep up with the weight gain from strength and conditioning while maintaining his playing ability (i.e., can't lift and play simultaneously due to many factors, most importantly being risk of injury). Go ahead and forewarn him that this is likely.
My son went through similar effort and results as yours after his freshman year. He now understands that he has multiple seasons, one of which is his strength and conditioning/weight gain season which is now (typically October through mid Jan). He will lift in between off days during season, but that's more to manage more than gain muscle. One season that is year round is proper nutrition. Caution him that when the baseball season begins, he needs to maintain healthy eating habits. This is critical.
Also, time of day of ingestion of the various proteins is important. One tip while he's in this heavy workout season is to drink a protein cocktail right before bed that consists of milk, protein powder, peanut butter and greek yogurt.
I assume that he has a good trainer/nutritionist managing/limiting supplements? bcaa, kre-alkalyne, glutamine, etc., and when to take (prior to/after workouts), etc. Also critical to maximizing healthy, managed weight gain.
Congrats on the success your son is seeing. It becomes addictive when he works hard and see's corresponding results. Great motivator.