
Well it seems pretty obvious that he doesn't like the car and I doubt that it is a recent thing.
You'll probably never know why he dislikes the car so much, so now you need to concentrate on calming him as you quite rightly said. Little but often to start with, short car journeys that lead to somewhere that is enjoyable for him, ie, going for walks to the park, woods, fields, etc. Any thing that he likes, ie, chewing a bone, playing with toys, being stroked, feeding, all of this in a stationary car with the door or tailgate open so there is no pressure on him and he doesn't feel the motion of the car. He needs to associate the car with
good experiences.
HTH