Phobia.

Mostly likley he was alone? He was mentally scarred, still traumatized by this accident, and the need for someone on the passenger seat indicates he needs constant reassurance. Needs to see a shrink as @ jamplu has pointed out to come to terms and put this accident behind him.

You can always Google the word.

I think it is normal.........I think he is still traumatized from his accident and feels safer when there is someone else in the car. I have the same fear too partly because i had an accident and partly because i am just scared of driving. Actually right after the accident,i would be on the passenger seat and keep 'instructing' my bro what to do and pointing at all dangers on the road-real or imagined,i could see a dog crossing the road from very far away .
Needs Help well maybe maybe not? well i do not think so,i think it will go away slowly......the more the pressure,the more irritating it will get for him. Now i can drive.....at a very slow speed,short distance and mostly in the village

Sad thing..he doesnt touch family car. 6yrs is a loong time not to have healed. True, he was alone when the accident happened.