He most likely needs to go see a doctor.
First get out of the sun and stay out of it.
This probably heat stroke or heat exhaustion. Be very careful, this can get much worse.
Heat stroke can be extremely dangerous, even deadly.
Heat exhaustion which this probably is, is slightly less dangerous but still must watched closely. Heat exhaustion can be treated at home (usually).
Heat stroke needs immediate medical attention.
If the symptoms are severe or if he has high blood pressure, heat exhaustion gets even more dangerous. If it progresses to heat stroke, get a DOCTOR immediately.
For heat exhaustion, get out of the sun and get to a cool, shady area.
Treatment of heat exhaustion:
Drink cool (not cold) non-alcoholic beverages.
Take a cool shower if he feels steady enough.
Don't wear lots of layers
However to be safe, if you believe this is HEAT Stroke instead, go to a doctor immediately.
Heat stroke symptoms:
Signs of heat stroke include:
* high body temperature,
* the absence of sweating, with hot red or flushed dry skin,
* rapid pulse,
* difficulty breathing,
* strange behavior,
* seizure, and/or
Good luck. He should stay out of the sun for a few days as well. And I suggest you see a medical person if any of the heat stroke symptoms appear.