A member of the boy's extended family left him in the car Wednesday, the Canadian Broadcasting Corp. reported.
It was not reported whether the boy was found dead in the car, or who found him.
Police said they were called to the scene at about 5 p.m. They said the boy had been in the car for a "significant portion of the day."
So far, an official cause of death has not yet been determined, but temperatures Wednesday passed 86 degrees with high humidity.