k2to DS was almost bound to get car sick as DH and I both were horribly car sick as children - and DH had to modify his driving so that I didn't get car sick (in the front seat) when we got together. DS, however, didn't get car sick until he was 3. it was like flicking a switch from happy, no problems on any length of journey (well, the occasional upset, but nothing on a regular basis) to being guaranteed to happen on every journey above 10mins. I don't know what it was that changed - same seat, same car, same drivers, same routes - perhaps it was just growing up and starting to properly focus on what was outside the car?
Because he didn't start until he was 3, we were advised to try joyrides by the Dr and have been using them ever since. They normally, but not completely, prevent the sickness.
Are you sure its car sickness and not simply having more to look at (eg you) when he sits in the front?