
If your pup is now afraid to go in the car, I would personally avoid it if you can. Instead, try desensitisation (hope I've spelt that right!). Open the back of the car, pop the pup in his crate, tell him he's good and get him out again. Proceed very slowly - you can do this half a dozen times a day, tho' not all at once. When he's happily sitting in the crate, go round and open the driver's door. Shut it again, and let him out. Car sickness is very much in the mind, and you need to break the link in his head. Proceed slowly until you can sit in the driver's seat and read the paper for 20 minutes with him in the back, settled down and probably bored. Then turn the engine on. Initially, turn it straight off again, ignore the pup for a few minutes until he's calm again, and let him out. Try to avoid too much praise or cuddles - a "jolly hockey sticks" approach works better. Cuddles can convince him that there's a problem. Carry on like this - your first drive will probably be only about ten yards before putting the handbrake on, turning the engine off and ignoring him again until he settles back down. I have done this with two dogs, one my own and one belonging to someone else, and it works, although you have to try and arrange not to take him in the car until you've been right through the retraining. It should take about four to six weeks - I know! But it's worth it.
Sheltpap is also correct - if it's just car sickness, not fear of the car, covering them up so the world doesn't rush past their heads will also help - and it's a good time for your best BSM driving - no sudden acceleration or braking, no swooping round corners etc.