CaCO3Girl, it's very common for kids your son's age to focus on strength instead of proper technique when exercising. That might be what the trainer saw. For example, if he was doing external rotation exercises he may have been lifting his elbow away from his side, which activates the deltoid muscle instead of isolating the posterior rotator cuff muscles. It's a natural inclination since that gives you more power. Shoulders rounded forward is another thing that lengthens and weakens the posterior rotator cuff muscles. I agree with you that you should check this out for yourself, and make sure that your son understands the guidance. (And make sure that you agree with it).