I like the leash idea.... I'm afraid the cat will run away. I have a small leash for my dog when he was a puppy. I think calling the vet is a great idea... don't know why I didn't think of that. My son says she's a good rider so I'm hoping I won't have 12 hours of cat crying.... don't know if I can handle that.
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NOT A COLLAR!!! Cats can get out of them easily, especially when frightened, even by a passing car. Use a snug harness...but even better, keep him caged the whole time he's in the car.
Go for drugs if the vet approves, it'll probably take a visit to the vet to get him to either prescribe or sell them to you directly.
Good luck...