DS was like that, I avoided the car as much as possible (but where we live, you HAVE to drive if you want groceries etc.!) and I'd also plan trips so that I could walk from store to store if I had multiple errands to run, even if it meant crossing busy 4-lane roads.
We also tried to plan to do errands/appointments as a family so DH or I could be in the back seat with him. Actually he is 2.25 and we are just starting to both sit in the front seat together sometimes again!!!
He hugely improved right around 9-10mos though. That was the same time we switched him to a convertible car seat, so I always wonder whether he just hated the infant car seat, or just got used to being in the car around that time? Guess I will never know... but if you could borrow a friend's convertible seat just to test it out, maybe your DS will do better with it and that will solve your problem??
DS also likes having a big huge mirror angled *just so* so that he can see my face in my rearview mirror. I sang non-stop the whole trip, anywhere we went, and that helped a bit too -- still helps when he's having a tough day. Shading the sun somehow (putting him in a baseball cap or adapting sunglasses to fit on him, for ex.) helped too because he's always been very light-sensitive.
I'll see if I can think of anything else...