Troy Whitaker, 41, was charged with aggravated manslaughter, sheriff's spokesman Kavontae Smalls said.
Whitaker left his Palm Harbor house Friday morning with his 5-year-old daughter and 23-month-old son, Lawson, Smalls said.
He dropped his daughter off at school, but forgot to drop his son off at daycare, Smalls said. He ran errands around town, and saw his son in the backseat of the Chevrolet truck in the afternoon, when he was back home.
Whitaker, who is a firefighter with the nearby Hillsborough County Fire Department, performed CPR on his son before medical officials arrived, Sheriff Bob Gualtieri told reporters.
The boy had an "extremely high body temperature" and was transported to Mease Countryside Hospital, where he was pronounced dead, the sheriff said.
"It was obvious that he had been dead for a while," Gualtieri said.
The sheriff said the boy was in the truck from about 8:30 a.m. to 4:30 p.m., when Whitaker returned from a trip to the supermarket.
"It was evident that he didn't realize that the child was in the backseat until he arrived home from Publix," Gualtieri said. "He was taking the groceries out of the front passenger seat and immediately dropped the bags -- there are groceries all over the driveway -- and then went and took the child out of the car seat."
The boy's car seat was behind the driver's seat, the sheriff said.
"As you can imagine, he was extremely upset -- visibly upset," he said.
The boy's mother was at the hospital Friday evening with other family members, he added.
Children more at risk in hot cars
An average of 37 children die each year from heatstroke after being left in vehicles, according to research by San Jose State University
. Already this year, 29 children -- including the boy who died Friday -- have died under those circumstances, the university said.
Since 1998, Florida has seen the nation's second-highest number of children's deaths in hot vehicles -- 72 -- after only Texas, with 100 recorded cases.
Children can overheat four times faster than adults, according to the National Safety Council
. Even in 70-degree Fahrenheit (21-degree Celsius) weather, a vehicle can reach life-threatening temperature in minutes.
Experts recommend parents place their bag or cell phone in the backseat as a reminder that their child is in the car.