If he gets carsick, you can call your vet to see what the proper dosage is but you can give them dromamine the motion sickness meds they sell for humans.
To get him more used to cars without giving meds you can try to slowly introduce him to rides (not sure if you have enough time for that before this trip). Start off by just sitting in the car with him in your driveway, after a few days, once he is comfortable with that you can then start the car up but don't go anywhere. Then back up to the end of the driveway. Start to drive to the corner, then around the block. Slowly increase the distance. Always make the car a happy place at first. Bring him to dog parks, pet stores etc... A lot of dog get fearful of the car because the only time they are in it somethng bad happens, like if you only bring him in the car for vet appointments, or make him sit in the car while you run errands etc..

Good luck & have fun!