Don't pressure the pup. Make sure the car is stationary with the engine turned off. Figure out how close he is happy to approach, and fuss and praise him. Then take a treat or toy and entice him one step closer. Praise and pet him, and finish for that session. The next session, start at the same spot, tell him how clever he is, then advance another step, praise and pet. The idea is to literally take it a step at a time until he is comfortable approaching the car.