It come be a few things. One, your dog is nervous about the moving sensation. Also, he doesn't feel stable and grounded.
Why Does My Dog Shake And Shiver When He Gets In The Car?
That's a sign of fear. Has he ever had a bad experience riding in the car?
Hold him, reassure him...he's just afraid...poor guy
Hold him, reassure him...he's just afraid...poor guy
If your dog is use to being in the car, and rides with you all the time, then I would have to say that he is being stimulated. There are so many things to look at and to see that in a sense, he is being overloaded with all of this, and just simply can't control his excitement. Hope this helps, good luck.