*Disclaimer* I am NOT a vet! I have a degree in Animal Sciences from an accredited university, but I am NOT a vet. (However my husband IS a lawyer, so I now feel the need for disclaimers, lol). With that said, I have experience with a car-sick pup and this is my advice/somewhat educated opinion.
I have no idea if anyone suggested this yet, but Dramamine. My APBT was CONSTANTLY nauseous (turned out to be a food allergy), so car rides were horrific. One typical pill for my 55-60lbs girl was all it took. Plus, a bit of Benadryl (25mg at first, add more if larger dog or they keep waking up. Hard to overdose on it, so it's okay to be generous) to help her sleep through it if it was a long ride. My vet was the one who told me how to use all of these meds, so I feel comfortable using and recommending them. If you can get it done with some more natural, like peppermint tea (which I also use) or ginger ale/ginger snap/straight ginger (have used for myself mostly), then by all means use those! But, none of that was strong enough for my poor girl, so she had to be on the "hard drugs".
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rayers for your pup's tummy::
P.S. Might be a behavioral thing too, make sure to eliminate that as an issue. If you have questions, feel free to PM me.