My car still has tale tale signs of a car sick gsdx. I would give him the non-drowsey dramamine which helped. He has outgrown it - 5 years old now.
A trainer I used very early on told me it was my driving. He made me drive him in my car. When we were finished, he explained to me that a human stomach flops up and down - that's why people get sick going deep sea fishing. He said a dog's stomach flops from side to side. He said I was driving from side to side and making him sick. I was careful to drive very straight. Whether it was that or not who knows but at least he has outgrown it!!