I asked the Vet about car sickness... she advised using Pediatric Benadryl... .25 of a teaspoon... (Alfie is 4 pounds).
On the way to the vet he didn't get sick.. for the first time. We kept him out of the carrier and held him.. this seemed to help. The only thing that did happen was he pee pee'd all over my husband...
The Vet said he may grow out of it.. but for now to try the Benadryl and then if that doesnt work we could find something else.