The father was sitting in a car at an auction house in the western Missouri county of Shawnee at the time, the Kansas City Star reported Wednesday.
The boy's mother and stepfather had gone inside the auction house when the youth went over to a car occupied by his dad and shot him with a handgun, investigators said.
When the mother and stepfather came to check out the noise, the boy handed the gun to them, authorities said.
The stepfather called 911. The father died at the scene.
Police said they have not determined where the boy had gotten the weapon.