i don't know anything about your son his medical history or have any way of sorting out the numerous reasons for his fainting but i can tell you that it is very important for him to be carefully examined including a cardiac work-up and lab tests before he runs again. there can be simple reasons like electrolyte imbalances hyperventilation etc or there can be serious causes like cardiac arrhythmias so make sure he gets a good medical work up.
why is my 19 year old fainting when he runs? . he gets heaviness in his ears and can't hear anything right before fainting.