Future Elbow Issues

My 16 yr old son plays high school ball - and has for 3 years as a catcher. He had pitched as a youth, and was successful, but has had the double-edged sword of attending a high school with multiple D1-bound pitchers in the two classes ahead of him, which left him with little opportunity to take the mound. With his junior year almost over, he has the strong desire to get back into pitching again, and so he threw two game sim innings this past weekend, total of about 40 pitches. His elbow is now experiencing some soreness and with some unintended analysis being available (his younger sister got a new camera with sequential shot and was taking shots of him as a test) it was discovered that after he separates and his arm starts to move forward he drops his elbow then raises it once again as he comes further to the front. You can see that his arm has almost a shotput motion, although not quite as deliberate. Long story short - I know that he will have nothing but arm trouble if he does not get this corrected. Any suggestions for drills/exercises to help improve his mechanics with this? Or will this just be a case of him needing to make lots of throws making sure he follows proper motion to help to build a proper arm delivery?