My son faints mostly when he is hot?

My son is a fainter. He has always been one to faint, once or twice a year since he was 5 years old. he is now 12. It most often happens when he gets really hot. It has gone on for 7 years so I do not think it's life threating but just looking for answers, it's getting embaressing for him and making me way overly cautious. This last time he fainted he hit his head and got a concusion. Has anyone been through this?