In line with several others' suggestions here:
(1) Strongly consider at least one summer session at the chosen college. The academic credit hours will be welcome when he starts juggling baseball time with academic time in the fall and spring; and he will be able to work out in the strength facility and with other players who are summering there. It also serves to help him get oriented to and somewhat settled in his new surroundings.
(2) Whether summer school is chosen or not, DO NOT show up in August worn out from summer activities; whether baseball-related or otherwise. Fall baseball will likely challenge him more than he's ever been challenged by a wide margin. The last thing he wants is to be fatigued going into it. He's about to find himself competing with college-veteran 21-year-olds who had equal or better credentials when they arrived on campus; and the combination of the physical demands on top of the academic (not to mention being away from home) can really take a toll.