Hi, I have a carseat as well for the pups. I recommend putting your pup in the car with you, go for a drive around the block, give him a treat. Do this a couple of times a week perhaps increasing the time of the drive and perhaps stopping somewhere like petsmart, this way he doesn't feel that the car equals vet all of the time ... he will warm up to going for rides. When Zander joined our home, it was summer so I would take him with us on the drive to drop off my son at the ymca, I would bring his dry food so he could eat in the car, he barely noticed the car ride. Since I did this throughout July he became a good car rider. Jazzie does well, but if I open the windows and she hears other big truck noises, motorcycles, she get nervous, so I give her a dose of Quiet Moment before leaving the house and it helps some.
Hope your pup comes around to liking car rides