How Can One Cure Lethargy, Tiredness And Weakness?

My son was diagnosed with anaemia last year and subsequent tests by the paediatrician could find no cause . He then went on a course of iron tablets and made a good recovery. This year he seems a bit tired in himself and sometimes seems a little drawn in his face . His colour (of his face) sometimes looks a little off to me but I can t say he is as pale as when he was anaemic .He plays a lot of sport and his trainer recently told me that he can t seem to run very fast which is unusual for him. He is now fifteen and has grown a lot in the last year. I don t want to worry him as he is reluctant to go to the doctor or am I just over reacting