My 5lb Yorkie still gets car sick and our Vet told us to give him a 1/2 tablespoon of Children's Liquid Benadryl. That seems to help us on long trips.
I also don't feed him, so his stomach is empty. I give him Nutrical and water along the way, per Dr's instructions.
I have tried to feed him, while on our trips but he doesn't have an interest in food as long as we are on the road.
I met a nurse and she gives her fur babies, a
lollipop dipped in Benadryl before she takes them out and that does the trick for her. Even if it's just up to the store for a quick trip.
Good luck with your baby.