First, I would ask If your son's coach is a certified personal trainer (and I am not talking one of those internet certificates) but AFA or FRA.
Next, I would see what type of liability insurance he has. Most personal trainers that are serious about personal training and do it as a profession have a minimum of 1 million dollars. This covers your medical expenses when your spine pops out of your back or you blow your knee out or rupture your spleen.
My guess is he has neither of these (the certificate/training or the insurance) and I would not be caught dead doing these exercises with him.
I know personal trainers that won't even let high school kids pick up weights until they have given each kid an individual evaluation that includes stretching, range of motion, balance, and several other tests that only they are qualified to do. They often discover problems kids have with their hips or back that they were not even aware of. Your sons muscles and bones are still in development and in reality, the only person who should be recommending weight training of any kind, is someone who is vastly qualified. If he has been lifting without a trainer, its a safe bet that he is using the wrong techniques, the wrong weight and the wrong type of rest in between muscle groups and any trainer will tell you that 99% of all high school kids fall into this group. If your son hasn't been lifting regularly, throwing 250 on the bench or squat bar doesn't really sound like a fun activity for someone who has not be working those muscles.
You can read more from an interview I did here with a personal trainer
http://www.varsityedge.com/NEI/varsity.nsf/main/weight+training+and+the+high+school+athlete