Sounds like you are making great progress with him. How old is your puplet ?
We've had a few abused foster puplets and would agree with all the advice given . Have found adult dogs tend to take longer to recover than pups.
With the car thing , once he is confident and trusting you , introduce him to the car and take things at his pace, but also not with him showing fear then you removing him. Or he might learn showing fear gets him results. As he is gaining trust and confidence by copying your other dogs behaviour I feel they will lead the way to showing him that car rides and walks etc are not as scary as he first though.