Hi Rosie - I'm sure that trying to strengthen the shoulder stabilizers will help - but unfortunately our young athletes will spend more time training the traumatic movement (the crawl in this case). I have had some success using KT Tape (kinesiotape) with some tennis players - overhead athletes as well - as this will somehow protect or even alter the movement patterns slightly. A good website with instructional videos is kttapeeurope.com. The pre-cut strips are easy to apply, and they even stay on in water, which can extend the protection to several days. Your athlete can even learn to use the tape on his own.