Suggest Breathing Techniques For Anxiety While Playing Sports

My son is 16 years old, in good shape, does sports actively. When doing a tennis session he complained about having difficulty yawning and taking a deep breath. He has earlier visited a doctor for similar symptoms but nothing diagnosed. Some web pages suggest such symptoms might be related to anxiety. Tennis can be a stressful game, especially if played competitively, as he occasionally does. He tends to be quite nervous before games, and sometimes loses games because of nerves. My question is, if this is about nerves, are there anxiety-relieving breathing techniques he could try to see if they work?