Welcome to HCM,
Your son"s pale look, and feeling breathless, first commonest cause would be Anemia
( Low Hb). I am sure that your GP might have rule out that possibility. There are several other causes, which we should also rule out, like Exercise induced Asthma
, Heart disease
, etc. Chest Xray may not rule out Exercise induced Asthma,and all the lungs and Heart diseases. He should also go for Lung Function tests. There should be a good clinical judgement.
Make sure that your son does proper warm up, and relaxation exercises also. His pulse rate also should not go beyond his target heart rate
. We calculate target heart rate according to his basal pulse rate and age. Your GP may help you in this matter, or you may give these details to us.
I hope, it will help you in further care of your son.