My son is not feeling well. he has a headache, and poor appetite. can it be just because it's so hot.

It happens: Heat induced symptoms from simple sweating to true heat stroke are possible in kids or adults. Headache & dizziness are often part of this range of problems. Moving the kid to a cool environment, providing fluids and cool compresses can usually stop the progression of symptoms. Progression to a 40c core temperature, fainting & seizures are all possible with heat induced illness.