Our 16 year old son trains in a weight room at a local gym. He is a left pitcher. Which exercises should he avoid to do?
Upright rows. And probably straight-bar bench pressing (substitute one-arm DB bench pressing).
He should focus on compound movements that use a lot of muscle - squats, deadlifts, bent-over / Pendlay rows, clean high pulls, etc. Omitting Olympic lifts like the clean and snatch are probably a good idea (high pulls are fine), though I doubt they’ll have him doing that.
avoid going too low on blast strap/gymnastic ring pushups, or anything that brings the elbows way behind the body
avoid the smith machine, and just about every other machine for that matter (some exceptions like reverse hyper, glute ham machines, but in general this is a good rule.)