Did you just get him or has he been getting sick since he was a puppy? Lot of dogs start out being car sick but get over it with time. My first beagle was terribly sick in the car for the first 5-6 months we had him and then over time he stopped completely. Molly also was very car sick from nerves when we first got her but now she is a great rider. My brother's new dog was getting car sick and he used ginger capsules which are all natural and they helped a lot.
Try some sort rides with a window open a bit. Try to be sure he doesn't eat, take meds or drink water right before the ride.