If your son is not into basketball, then I think off season baseball conditioning and workouts are fine. Everything in moderation especially during the high growth years. If baseball is truely his passion then go for it.
Patriot Son has played competitive basketball since 5th grade. Multi-sport training has really given him an advantage IMO. He has not done alot of serious off season baseball training but he is in excellent shape.
Last year he played 14u Babe Ruth that started in April. The level of play improved when the HS Freshmen joined in May. Patriot Son is a LHP/1b and throws low to mid 80's. I believe that his core strength has benefited from his basketball. This year he is playing Freshman ball and usually does not come out of the game. That's a good workout.