Investigators said the boy's father drove home with the child, left the boy in the vehicle as he went inside, then left the home but drove away in a different vehicle.
When the unidentified father realized the mistake he called authorities, KTLA-TV, Los Angeles, said.
"The parents initially realized the child may have been left in the vehicle, called Carson station, and deputies responded," police Lt. Dave Coleman said.
Police found the boy in his car seat in the vehicle. Paramedics worked to revive him but he was declared dead at the scene, KTLA-TV said.