Does he start complaining when you start driving/start the car, or when he's put in the car seat? If he does it right away with the seat he might be uncomfortable? I know reflux babies are very sensitive to angles etc. Could try adjusting the car seat angle with folded towels underneath it (since the natural slope of the back seats vary between different cars, the angle of the carseats can vary a lot! Also can take the carseat out and have him sit in it in the lounge where he can see you to get used to it.
If it's the motion of the car itself, I can't be of much help. I loved the head rest inserts that stops their heads from going side-to-side, and had a toy to play with, but my son thankfully was pretty content with me in the back for the first few months, then on his own once I could drive again.