I had the same problem before with my dog (about around your dog's age). My vet told me I can give him half a tablet of dramammine 2 hrs before he gets in the car. (no food 2 hrs before either) For longer car rides I was able to give him a whole tablet. Try taking your dogs for shorter rides and extend the trip longer as he feels more comfortable. Keep him away from the window (if you can) so he won't be able to see how fast things are going and gets him really dizzy. I have tried covering the back passenger windows with towels. My dog used to start drooling as soon as he gets in the car, but since I've taken him out for rides, he automatically jumps in the car when he sees the door open and is ready to go!
Hope this helps!
