, Welcome to JustAnswer. I'm a licensed veterinarian and I'll be happy to help. Fear of and anxiety during car rides is actually pretty common. . .poor Chewbacca. The best way to handle this is to desensitize him to the car rides, recondition him to the car rides and actually be a little sneaky. Here's what I'd recommend:
Start taking him on very short rides with good ends and then give him a treat. At first just drive him down the driveway and back. I'd repeat this several times over a day and always follow with a treat. After this, you gradually expand his trip (around the block, around several blocks, etc.) and again follow with a treat. Finally, take him somewhere fun (or at least not horrible like the vet's), include him in many car trips so that when you do have to go to the vet, he won't know at first, and of course always follow with a treat. Even at the vet's he could get a treat when he gets there and after he gets home.
All of these treats could get fattening. Many dogs love baby carrots , so you could try that. Of course when you give the treat always praise him job well done.
Hope this is helpful! Any further comments or questions? I'll be here most of the day. Dr. Barbara