I would get an eval from a PT who works with athletes and lower body injuries. My concern would be that he's putting undue stress on his knees. Your primary care physician might write a PT script for this, or you might have to pay out of pocket. That doesn't mean he needs PT, but he may need to do specific strengthening exercises to protect his knees and hips. IMO, rigorous training should be monitored closely for kids who are growing fast, and it sounds like his trainer is doing just that.