Suriyakarn Thakan was found unconscious inside the cab of a pick-up truck which was being used to transport children to a nursery school in northeastern Si Sa Ket province bordering Cambodia on Tuesday afternoon.
"For two hours doctors tried to resuscitate him... but they couldn't," Lieutenant Colonel Thinakorn Dechalert of the local police told AFP by telephone.
It was due to be the child's second day at nursery.
Police said the driver of the pick-up has been charged with negligence resulting in the death of another person, while the nursery teacher who owns the vehicle would be questioned by police on Thursday.
His death follows that of a three-year-old girl who was found in a coma after being left in a school bus for four hours outside Bangkok in early April.
She died in intensive care a fortnight later, prompting police to charge a teacher and the bus driver with gross negligence.
The cases have sparked an outcry over lax child safety standards in Thailand, where summer temperatures routinely soar beyond 35 degrees Celsius (95 Fahrenheit).