S.C.
I also suffer from motion sickness. It has gotten better, but I still have it. Try to crack the window in the backseat. Have him take off his coat and keep the heat down. No reading or playing video games in the car either. Avoid jerking to a stop and gunning the vehicle when you take off. Also watch out for curvy roads. I know these are all things that help me. Also, could he sit in the middle in the backseat? It may help to be able to see out the front window. I'm not a doctor, or a motion sickness expert, I can only pass along what helps for me. I hope some of the tips work for you and your little guy.