The body of Ennis William Cosby, aged 28, was discovered lying by his dark green Mercedes convertible by a woman driver who stopped to help. Mr Cosby died of a single gun shot to the head. He had apparently pulled over to change a tire.
Comedian Bill Cosby, 59, rose to fame as a good- humoured family man in The Cosby show. He and his wife Camille, the parents of Ennis and four daughters, were told of the death early yesterday in New York City.
"We have every confidence in the LAPD. This is a life experience that is truly difficult to share," he said. Of his son, who was studying for a second Masters degree at Columbia University, New York, he said: "He was my hero."
Mr Cosby's car was found with its emergency lights blinking near an exit ramp from the San Diego freeway, one of the main road arteries for Los Angeles, in the Santa Monica mountains conservation area. The freeway links west Los Angeles to the urban boroughs of the San Fernando valley.
"The victim was apparently having car trouble and stopped to change a tyre," a police spokesman, Lt Anthony Alba, said. "The car was found with the trunk open and the door open on the passenger side."
The woman witness saw a "portion of the incident" as she drove by at about 1.45pm, police said, and provided a description of a possible assailant. She was still being interviewed yesterday.
Police said no property appeared to have been taken and declined to speculate on a possible motive.