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If you have the time. Get him in the car, eventually, leave the engine off and sit for a while then get him out and go and chuck a ball about or similar. Just build it up slowly so that not every time in the car is the same but keep the stress low. Even if you only drive around the block and nothing bad happens that is a positive. My old dog was two and a half and wasn't car friendly but he wasn't stupid and knew quickly that the car led to good things. Also, if it works out, feed him after your drive or training never before. He's less likely to throw up on an empty stomach and he'll associate the car/walk with food.
Yeah, we currently have full-bodied resistance to getting into the car and a highly-stressed panting once in it. I think maybe he actually gets car-sick as there have been a few sicky incidents, but he's managed longish journeys recently without sick so maybe not that. He palpably hates it and feel awful getting him in against his will. He seems to go back to his normal sunny self pretty soon out the other side and the last two trips (riverside getaway and my folks' place just outside Oxford) he's LOVED once he got there....