Police were called to an address in Sneydes Road in Point Cook at about 3.15 following reports a child had been left in a car alone.
Paramedics arrived and treated the boy at the scene before transporting him to the Royal Children’s Hospital in a critical condition.
It is understood the vehicle was parked outside The Brook Hotel.
Temperatures in Melbourne reached up to 36C today.
Police are investigating the circumstances surrounding the incident.