Police aren't identifying the suspect because of his age. But they say he approached the car driven by 58-year-old Michael Schacht, who was slumped over and in distress.
Police say the boy pulled Schacht out of the car, stole his wallet and then drove off, leaving Schacht on the ground.
Police were puzzled by Schacht's death because he had only a superficial head injury when he was found April 10.
The suspect doesn't qualify to be charged as an adult. He was being taken to a juvenile detention facility Tuesday.