The juvenile was helping the 47-year-old man at his home in the 300 block of Newman Drive when they got into an argument over a stereo amplifier the man said the boy could have for his assistance, South San Francisco police Sgt. Joni Lee said.
The argument turned physical, Lee said, and the boy choked the man until he was almost unconscious. He then took a hammer and struck the man twice in the head.
Police responded to reports of a disturbance at around 12:30 a.m., Lee said.
The boy was arrested on suspicion of attempted murder and booked into Hillcrest Juvenile Facility.
The man was taken to Kaiser Permanente in South San Francisco with serious head injuries and internal bleeding.