My dog is always very nervous in the car. Lip licking, yawning, and shaking. We always take him fun places, and he'll get in on his own, but I know he hates it. We've tried giving him treats, but he wont take them in the car.
I'm not sure if this is THE answer, but I can tell you what has helped me. I'm assuming your dog is not in a kennel while in the car...and that you drive to places for walks? I have no room for a medium to large dog sized kennel in my car, so the dog gets the whole back seat. I've owned a lot of dogs throughout my middle-aged life and find that each dog behaves differently in the car, but most dogs will eventually relax in the car after so many times of riding in the car with you. Some dogs are really anxious about what they probably perceive to be the "bottomless pit" below the seat where human passengers would put their feet. I have solved this issue by purchasing a dog car seat cover that acts like a hammock/sling to cover the "pit". The cover/hammock/sling makes the dog feel safer, and sort of blocks the dog from jumping into the front seat from the back seat. I always check in with the dog in a low, calm voice so they don't get too excited. Also, don't play music or the radio really loud because that by itself can make a dog nervous. Slamming on the breaks and fast turns can bump the dog around, so avoid whenever possible. Basically, I drive like I am driving my sick grandma to a doctor's appointment! The car seat cover/hammock/sling has worked wonders for me, and even has slits in it for seat belts if your dog would tolerate that (makes ones that do feel really safe). The seat cover is washable and easy to install. I got mine at Amazon.
My dog also worries more when in the back seat of a sedan. She shakes and pants, and the only thing that seems to help is my older dog pinning her to the back of the seat with her body, which she likes to do anyway. She's much better in my hatchback with the seats folded down so she has a bigger, flatter space.
Most dogs get nervous due to the unsteadiness of the ride. Getting them to lay down on something familiar in the back seat is ideal or even opt it getting them a seat harness! Talk to him and keep him calm.
You're doing a really good job of making it a happy place -maybe every now and then try putting him him it and not going anywhere, and just sit for a couple minutes. If it gets really bad, call your veterinarian and ask about some anti-anxiety medications!