How long have you had him? He might just need to get used to you, your car, and realize that you're not getting rid of him.
Lola threw up 3 times in 3 hours on the way home when I got her and a few days later she threw up twice during a 5 minute car ride.
I didn't take her out again until she needed to go to the vet a week after getting her, she shook but didn't throw up.
It took a few more rides to get her comfortable, but now she does great. I try to take her out at least once a week to keep her used to the car. |