Reading all the above advice..which is good. If he does not fall into any of those areas, my guess is that he has a real case of 'stage fright'. At 13, he is really at an akward age and he needs some confidence building. When he is sparring, gather a small group to yell and try to distract him during training. Basic cure is to have someone speak before an audience. Years ago I head of a student that was up and coming with a lot of promise, and at his first Amateur match, shook so badly that his teeth chattered and they had to scratch him from fight. Never knew how he came out. If he can spar before a small crowd, that may solve it. If indeed it is acute stage fright.