Size is projectability. His size will get him about 3-5 mph in a coach's mind. So don't let him get frustrated with the velo in itself because his size will give him an advanatage. My son was never tall and didn't have the velo but had the pitches and pitchability. Everyone is given something and you have to use what you are given. At 15, he may be able to grow a couple of more inches.
As parents, we can only do so much but maybe next time talk to his ego and remind him that the leg workout takes away velo and the velo is what the coaches are looking at first and foremost. Son did a leg workout the other day that was required and he said he knew it affected his velo 2 days later as a 21 year old.
Just remember that your role is just as vital as anyone in the process. He always needs a cheerleader in his corner no matter what else you provide.