The teenager punched Richard Mannington Bowes, 68, knocking him to the ground where he banged his head.
Mr Bowes was taken from the scene near his home in Haven Green, Ealing, west London, but died in hospital.
The boy admitted manslaughter when he appeared at Inner London Crown Court today beside his mother, who is accused of perverting the course of justice, which she denies.
Mr Bowes died on August 11, three days after being attacked while trying to put out a fire in a dustbin near his home.
He suffered brain damage when his head hit the pavement.
The defendant, who cannot be named because of his age, also pleaded guilty to violent disorder.
He had previously admitted burglary at William Hill, Tesco Express, Blockbusters and Fatboys Thai restaurant on August 8.
He is accused of murdering Mr Bowes and will reappear at court tomorrow with his 31-year-old mother to stand trial.