At the minimum consider massive hydration, ideally with high electrolyte solutions and only a little bit of carbohydrates.

Where sunblock, loose fitting clothing, avoid being outside between 11-4 PM if possible.

There is some genetic testing as well.

Getting heat stroke is typically a sign of extreme weather, dehydration, and extreme exertion mixed together. Step one would be to determine why your son is passing out because it may not be heatstroke but other cardiac issues. Drinking plenty of water before and during the activity is best. Also, proper nutrition is a key factor as well. Limit the outdoor activity during the hottest parts of the day 11-3. I'd recommend seeing a Sports Physical Therapist and getting a VO2 max test done to determine his cardiovascular strength.

If he has experienced multiple heat strokes he needs a medical workup

1) hydrate
2) hydrate
3) hydrate