G.A.
I am so happy that you are being able to slowly help your son with his fears.
I know this feeling all too well as I was in a head-on collision at the age of 7. There was only 15 inches left between the front windshield and the front bumper. For years when in a vehicle if I thought we were too close to something or that we were going to hit.... I would slam my foot to the baseboard of the vehicle as if putting on the breaks. I would have flashbacks for years as I was able to see us actually impact. It was not until just this year (31 years later) that I got my first driver's license.
Take whatever precautions you can to help your little guy so he does not go through a lifetime of torment.
Sending you my good wishes and prayers that you can help him.