Have the vet check out your dog to make sure he doesn't have a health problem. Your goal is now to get the little one comfortable in your car. So make it a game.
Step one is to practice getting in and out of the car, while it is parked. Coax your dog into the car and give him his favorite treat. Then let him out and praise him. Do this several times and over many days.
Only after he is doing that with no problem, should you take him for a ride. The first ride should be short, just around the block. Give him treats while you are riding. Do this until he seems ok.
Then gradually increase the car rides. When you do start going for rides, go to fun places. Don't always go to the vet, or he won't like it. Go to places where he will associate fun with the car ride.