Everyday,
I swear I have posted this a million times but here goes again:
Sprint and speed trainers will give you a phased lifting program that matches the season you are in. For sprinters right after season with a little recovery time they go into a heavy weight lifting phase for x months, they then transition to an explosive phase, and then they taper down to maintenance phase during track season. They don't do a lot of running other than some technique work during the heavy lifting phase. When they get into the explosive phase they start to work on actual sprinting work. This is all done when they are fresh and they stop when there is any fatigue since it develops bad technique.
Baseball players should do the same thing, and they should be winding down their heavy lifting and moving into an explosive phase soon leading up to the season practice start in January. My point is that speed development is as much about lifting properly as well as running.
If it was me and my son had not been on a program, I would get him on a short heavy lifting phase now, probably going into December and then switch over to the explosive stuff.
Here are some good national references. I swear I should be getting commissions from these guys, but I have no vested interest in any of them other than spreading the word on how important weight training is to baseball.
I would start with eric Cressey.
Good luck!