The boy had been looking for sweets and gum when he set the car rolling down the driveway.
Dad Pratik Pandya, 48, rushed to try stop the VW Touran but he was crushed against the wall of his driveway, causing severe abdominal injuries and a deep cut to his left leg.
West London Coroners' Court heard Mr Pandya, an IT consultant from Isleworth, died two days later in hospital after three operations to try to stop internal bleeding.
He was preparing to go shopping with his son and his accountant wife when the accident happen on May 5 this year.
Reading a statement made by Mr Pandya's wife Nikhila, deputy coroner Elizabeth Pygott said: "She could see the Touran rolling backwards with her son sat in the middle.
"Her husband was not sat inside the vehicle when it was rolling, and the driver's door was open. Her husband was half in and half out of the vehicle.
"She could see he was attempting to do something in order to stop the vehicle, but was looking confused and unable to know what to do to stop it."
Mr Pandya had spotted the car rolling away as he went inside the semi-detached house to collect something, and rushed outside to try to stop it.
Jane Carne, the first paramedic on the scene, said: "He tried to stop it but got caught between the wall and the car.
"There was a lot of blood coming out of one of his calves on his left leg, and I could see a lot of blood in the foot well of the car."
Mr Pandya was taken to West Middlesex University Hospital where it was discovered his colon had been severely damaged. His condition deteriorated despite undergoing three operations for internal bleeding. He died two days after being admitted to hospital.
The coroner said Mr Pandya's son should not shoulder responsibility for the freak accident as she recorded a verdict of accidental death.
"It is not for me to say, but it is not apparent there is anyone to blame for this, especially Mr Pandya's young son," she said.
"It is plain to me, very strongly, that this was an accident. It is not something someone could have prevented, it was untoward."
Recording a verdict of accidental death, Ms Pygott said: "In trying to prevent his car from rolling backwards down the front drive on to the road, he became trapped between the driver's door of the car and a brick pillar at the top of the drive.
"It caused severe internal injuries which were not amenable to treatment."