I just give my girl her favorite chew treat & that keeps her occupied. You might start just going out & sitting in the car w/him & letting him chew on a treat so he associates the car with yummy treats instead of yucky vets, groomers, etc. Maybe for 5-10 minutes 3 times a day then take the treat away when you go out so he looks forward to getting in the car for the treat, then after he's comfortable just start the car but dont drive, let it run for a bit then turn it off & do it until he has no reaction, then you could drive around the block, & then 2 then 3, until he is ready for the big trip
Good Luck. Some goods meds wuld help to, I get car sick if I dont drive, maybe if he drives
