This clastrophobic reaction can have a history starting back in time when your dog was separated from his mother, and a sheer smell of the car may revive emotional state. Get a trainer with his dog assisting him in training. The assistant will jump into your car and your dog will follow him without any problem. Leave all detailes to the trainer. Many dogs enter water, train, etc without fear being led by another dog with ease, they stop chasing joggers, reacting on naughty children, looking at cats, they learn from another dog much faster all good and bad habits which we, humans, may try to install in their brains for days and months without much success.