You are on the right track with trying to desensitize him. You may want to start with just sitting in the car on the driveway once a day for a week for just a minute. Then a trip around the block daily. Always finish with high praise and a reward. Very slowly build up his tolerance until you are off and running. Alternatively, there is a new anti-nausea drug called Cerenia that shows promise with car sickness; discuss availability with your local veterinarian.