In season lifting is all about maintenance, so his routine should be a circuit type with a couple of leg sets, back arms, chest and core. Lighter weights higher reps. No overhead of any type as you can get shoulder impingements that can effect throwing. I would consider a couple of yoga or pilates classes a week with 2-3 lifting days, if he can fit it in. It gets difficult for a HS player to make it happen because of time constraints.