Goose nailed it! I have seen far more bad then good training in HS weightrooms with most of those run by the sport coach. I have no idea why but it seems like every coach who has ever lifted weights believes that they are qualified to run a strength training program. As Goose and others recommended, find a reputable, well-trained performance training coach and have him work with your son on developing correct movement patterns such as the hip-hinge.
Once he starts working with the high school coach, see if you can get a copy of the training program or have your son take pics of the workouts. Then share with outside coach to confirm that it is safe/effective.
Disclosure: I have a grad degree in exercise science and have 10+ years of experience working with high school athletes in a high school setting.