Adolescents and young adults who are active and use their arms in repetitive and forceful motions can develop problems with the elbows. It's for this reason that many organized sports, such as some baseball leagues, draw up rules to reduce the risk. For example, many leagues have "pitch counts" that only allow their pitchers to pitch so many times within a given period of time.
Proper conditioning will help keep the limbs strong and reduce the chance of injury. If your son has any signs of pain or discomfort in his elbow, he should rest it and not play through the pain. He should also see a doctor if he has any recurrent pain, difficulty moving his arms, extending his elbow, or if he hears clicking noises from the elbow.