My Son Has An Issue With Overheating When He Is

My son has an issue with overheating when he is working out or playing sports. I can see his face turn red and then he will go into heat stroke. I have done every thing from pre-game hydration to special diets. I'm not sure if it still could be related to his heart murmur he had as a child. His current physician doesn't believe it is the issue, but cannot give any helpful advised how to combat this. No sure if we should be looking for something else we're missing??