It happened at 5:45 p.m. Tuesday on I-76 West near Spring Garden Street in West Conshohocken.
Police say a family of three - the father, his son and another family member - were traveling in a 2010 white Buick Lacrosse.
At some point, police say, the son, who was seated in rear of the vehicle, shot his father, who was driving.
The bullet struck the victim in the head.
The 42-year-old Philadelphia man managed to stop the car without hitting other vehicles.
He was treated at the scene by paramedics who happened to be in the area at the time.
They took him to Penn Presbyterian Medical Center in critical but stable condition.
The 22-year-old son was arrested at the scene and taken to the PSP Philadelphia Barracks in Wynnefield, where he was questioned.
No charges had been filed as of early Wednesday morning.
Investigators say the weapon was recovered at the scene. They gave no indication what may have led to the shooting. null