Authorities say the boy's father found him head-first with his legs sticking out of a water-filled top-loader at their Castaic home last Wednesday night and pulled him out.
Los Angeles County sheriff's officials say the father had been working in the yard and the boy's mother had gone to the store.
The boy wasn't breathing when firefighters arrived. He was resuscitated and hospitalized but died Sunday. His name wasn't released.
Sheriff's homicide detectives are investigating the death.