Dog motion sickness is more commonly seen in puppies and young dogs
than in older dogs, just as carsickness afflicts more children than
adults. The reason for this is because the ear structures used for
balance aren’t fully developed in puppies.
If the first few car rides of your dog’s life left him nauseated, he may have been conditioned to equate travel with vomiting, even after his ears have fully matured.
Stress can also add to travel sickness, so if your dog has only ever ridden in the car to go to the vet, he may literally worry himself sick on the road. (1)
In your case it appeared that your dog became habitual to the motion sickness even into adulthood, may be because of stress etc. Its similar to humans sometimes few people became habitual to motion sickness.
The best way to prevent dog travel sickness is to make the car ride as
comfortable as possible for your dog.
You can do it by giving him comfortable ride as much you can. Half opened window can be helpful to pass air through ear. Giving him company in ride rather then driver can be helpful. Try changing his seat from front to back or vise versa ( or change car if possible). If nothing helps then veterinarians help can be taken, there are some medicines for it too but which should be taken only after doctors advice.
For more tips got to (1) Dogs and Motion Sickness pets.webmd.com