I can tell you without a doubt that it would be impossible for a baseball player to throw a ball without the rotator cuff (actually 4 muscles) engaged. This trainer either has no idea what he is talking about or doing a very bad job of explaining the issue so that your son can understand. I would hope that he was trying to say that your son has shoulder laxity/instability. There is obviously a ton of stress put on the shoulder when throwing. If there is a weakness in the rotator cuff, there is going to be less stabilization of shoulder socket....leading to potential injury. This is obviously very simplified way to explain but highly recommend reading anything by Cressey & Reinold. I also like the recommendation JCG.