Same issue here.
Cerenia. Works wonderfully. Ask your vet for some for long all day trips.
For short trips, don't feed him. I had to put Seger in the front seat. The motion in the back it so much for him.
Luckily, my breeder said any others that had motion sickness outgrew it.
Seger vom Sitz von der Hose
Forever loved and missed.....Banshee
~The strength of one's opinion should not exceed their knowledge on the matter~