both of my daughters for awhile got car sick.. I kept the oldest one (the one with the bigger problem with car sickness) backwards facing for 3½ years before we finally gave up and moved her forward facing..
What we always did was keep the car cool, turn on the AC and blow it towards the back so that they get plenty of cool air, put shades over the window to try to block the sun as much as possible, we always had something for them to snack on (I don't know how old your son is, if he is snacking on stuff or not) we had crackers or something that was fitting for their age, it seems like if they get to munch a little it settles their stomach more.. If you can plan your trips around nap time of course helps a lot.. It was always better if they had a little bit in the stomach before.. We tried both with eating something smaller and eating nothing before trips and they seem to do better with a little something in it.. We would give them toys to play with that they could move around and play with. A lot of people say don't have anything for them to do, but we found they did better if they could get their minds off of the scenery going by. If there is someone extra in the car it's good if they can sit back there with him.. you can sing to him and try to keep his attention and entertained, plus you can tell when he isn't feeling good and then take a break.. get out of the car and get some fresh air.. better that a trip takes a little longer than having to stop anyways to clean up puke.. Always keep some wipes and bags close at hand and an extra pair of clothes in the car..
Some recommend those Sea bands, I could never get them to stay on their wrists, so I cannot say if they work or not, but a lot of people that swear by them.. I got some medication for my daughter from the doctor but she was over 1 year old and I'm living in Sweden so I'm sure there are differences in what you can get a hold of.. but you could always talk to your doctor and see if there is anything you can do..
Good luck I hope you can figure out what works for you..