A firefighter has been charged with aggravated manslaughter in connection with the death of his 23-month-old son, who was left in a truck for eight hours.

Troy Whitaker, 41, put his son Lawson and his five-year-old daughter in his pick-up truck on Friday morning and took the girl to school, the Pinellas County Sheriff's Office said.

The Floridian firefighter then drove home without taking his son to daycare nursery, and left him alone inside the vehicle until around 3.30pm, when he drove to a shop.

He did not notice that Lawson was in the back seat of the truck until an hour later when he got home and began unloading the shopping, police said.

The Hillsborough County Fire Rescue firefighter attempted to revive Lawson after calling 911, but the young boy died in hospital.

Pinellas County Sheriff Bob Gualtieri said Lawson's body temperature had reached around 42 degrees celsius.

Whitaker told detectives that he thought he had dropped his son off at day care in the morning, and had driven home to study for an exam at the fire department.

"You've got to be responsible for your kids. You can't leave them in a hot car and let their temperature get up to 108 degrees (fahrenheit)," said Mr Gualtieri.

"He did this with negligence and, under Florida law, that is child abuse. He has got to be responsible for his actions."