the question is when the car accelerates the atmosphere inside that packed car also accelerates due to the inertia but in a car (with height more than 10 feet) in which superman is in no contact with the base or any other part of car - would he also accelerate with the car without any effort on his own due to the packed car environment or he would be struck with the back of the car as he remains at his position and the car moves ahead?
If superman were suspended (not subject to the laws of gravity) inside a car, and that car starts to accelerate, he would experience a small amount of force (from the air starting to move) before smacking into the back of the car.
Interesting (and peripherally relevant) experiment: if you have helium balloons floating inside a car, and you brake, the balloons go backwards (they "sense" the pressure gradient due to the deceleration, and that causes them to move). See this article for a bit more detail.
I would answer like this: Every time a lifeform with the density and weight of a human gets into a car which then accelerates, the lifeform is "hit" (ie. is pushed back into) by the back of the seat he or she is sitting in. Therefore, assuming superman is the same density and weight as a human lifeform, then he will similarly be hit by the nearest thing in the car behind him when it accelerates, in his case not the back of a seat, but still the nearest solid thing behind him.
It's a simile under deceleration when the dense air "hits" the front of the inside of the car. Humans vastly denser than air under deceleration would, but for the seatbelts, brush aside the air and take the prime position squashed at the front of the car.
A helium balloon, less dense than anything else that can move inside the car, is simply pushed out of the way by all that denser stuff (air, people, who knows, maybe the family dog...) heading towards the dashboard in obedience to Sir Isaac Newton. So the balloon goes "backwards" because the front is now "full up" with heavy stuff and two things can't occupy the same space. QED.