Random note: You say the kid plays baseball, but the title of this thread is "basketball". No big deal though.
First, I'm a little scared to ask, but exactly what is the team workout he's doing, and how often is it?
As to helping his power, I'd look at his swing technique first. Pivoting his feet at the right time, following through on the swing, all the "basics" like that.
For exercises, generally speaking: Dumbbell squats, chin-ups, push presses, russian twists or full contact twists, rack walks, and dumbbell swings or snatches. However, I wouldn't necessarily jump into some of those. It depends a lot on his current lifting ability.