We have a big problem with our oldest in this area. Every time we went more than 30 mins from home, we would have to pack the car completely full with towels, bottles of water, face washer, spare clothes etc etc etc. No room for the rest of us! lol.
One thing we have found though, is that in a car, and I'm talking your standard car where the seats are much lower than the windows, she got sick all the time. In the ute we used to have, because the seats were higher, she never got sick (execpt for one time after we stopped at the petrol station and got fumes all through the car, but I was sick too! lol) because she was better able to see the world outside.
The problem usually stems from them trying to watch what is happening out the window and not being able to see properlyl. The lower the seats in the vehicle, the more likely they are to get travel sickness because what they are seeing is whizzing by really, really fast. The higher the seats, the more they can see things like the other cars going by, and the things further off the road. This helps to keep their equilibrium (balance) more in line and they are better able to control their stomaches.
I would recommend a booster seat, the big funny looking foam ones not the small cowboy looking ones, as they are higher sitting and just as safe. This will allow him to better see the world outside, even if he is just in a standard sized car.
Funny thing though, a child that is travelling for long distantances soon after birth, and I mean on a regular basis, will have less problems with travel sickness because they have grown up learning the control. True for our kids, our first is terrible with travel sickness, but our second and third don't have a problem. Haven't travelled far with 4 or 5 yet!