There is great information in this thread I can add just a few things.
YGD I would assume your son’s program has a strength and conditioning coach who would put a program together for him, including recommendations on how and when to creatine load, as well as a program tailored for a pitcher, which would likely be lower body focused in combination of yoga or pilates.
To put on mass you need to:
1. Have a great program that is structured in phases, first general conditioning, second body mass building, third dynamic/explosive/athletic movements, finally in season maintenance.
2. Probably as key to anything you have to get in the weight room and work very hard when there. Lots of kids just hang out and sorta do the work. This is probably more important than anything else.
3. Good nutrition is just one part of the program. My son did do a couple of “courses” of creatine, in which he put on more weight, but a lot of it was water and it came off after he finished his loading period . He put a lot of body mass on the past year and a half, but this was because of the totality and effort of his program, and I am not sure how much the creatine helped. It probably did, but I am not sure how much.
He used all kinds of different proteins, but found that a good and cheaper source is the Cryto Sports stuff at Cosco was as good as any other. One key for him was to spread his protein before and after his workout and found that “in workout” energy source drinks helped him keep up his intensity while in the weight room. His creatine loading was always done when he was in his mass building phase, which frankly is over the summer and early fall. He takes amino acid supplements separately so he does not need all of the blended proteins (and more expensive). He is now in his explosive/dynamic phase getting ready for the season. I am not saying that your son could jump into a mass building phase now, but he is out of sequence as far as the time of the year. You do lose some explosiveness after a mass build phase and you have to work to get it back. As others have posted you need to drink tons of fluids when on creatine. My son found that the “pure” creatine was the best and most economical.
There are all kinds of mixtures, just get 100% creatine and follow the loading instructions to the T, and like others have said drink tons of fluids. He will put on body weight initially, a lot is water, and some of it will come off once he stops the cycle.
Hope this helps and best of luck to him this season!