C.R.
My son is now 4 but has the same problem. For the short trips, I give him 1/4 tab of the children's Dramamine and for longer road trips, I give him 1/2 (which makes him drowsy). He weighs 32 pounds, but I have given it to him when he was less than 30 pounds and he does fine.
As for the carseat - I use old towels and cut holes in them to fit around the lap latch and keep several in the car. If he throws up while you are in route - it's a quick fix.