Speaking from experience, I'd highly recommend that your boys do exactly, and do everything, what the PT says to do. This can be a very tough injury that will stay with them for years. About 12-14 years ago, when I was in my late 30s, I tore my upper hamstring while stretching before a karate class. I did not get proper medical attention in a timely manner, or go to physical therapy, and the effects from that injury still occacionally cause me discomfort. At 51 years old, it's no fun to have "a pain in the butt" literally, so take care of it. I tore mine up high near where the tendon is, and if I had it to do all over again, I'd have gone to the doctor right away, and done everything he and a PT would have presribed.