I fainted once when I had the flu/food poisoning. I fainted once after giving blood. I almost fainted once while hiking in a high mountain area in Idaho. Now I am sure that it was caused by dehydration.

Choir directors warn singers not to lock their knees, because that can lead to fainting. That might be what happened at the concert.

The second incident might have been caused by lack of water or lack of salt, but I would expect the ER people to catch that.

If I were you, I would forbid bike riding in really hot weather and watch your son carefully for any further signs of trouble. You can't risk him getting woozy on his bike when there might be cars around.

Make the prohibition on bike riding apply to the whole family so he doesn't feel singled out.