My daughter was like that too. Whenever my husband drove, I'd sit in the passenger seat and stick my pinky finger in her mouth for her to suck on... she wouldn't take a pacifier. Oh, I don't miss those days!
This too shall pass... it might mean that you don't go out as much for now, but it will get better eventually. As long as he's not hungry/cold/hot/wet, his needs are accounted for, so if you have to go somewhere, unfortunately, he'll just have to cry. I know it's heartbreaking, but I don't think there's much you can do other than limit the amount of time yo'ure in the car until he outgrows this.
Oh gosh, I just read the other responses... please do NOT consider turning him forward facing! He is much too young and that would put him at risk of a broken back, a broken neck, or worse. Babies do not have the neck/head strength to sit forward facing in the car until they are at least a year old. Even better to keep them rearfacing until they outgrow the limit on the seat... I kept my daughter rearfacing until she was 2 and 30 lbs. Here is an article with good links including videos on WHY they need to be rear facing: http://babyproducts.about.com/od/carseats/qt/rear_facing.htm
I'm not sure if you were considering turning him around or not, but I just wanted to let you know that you'd be putting his life at risk if you did... and I'd urge anyone else who has a small child to keep their baby rear facing for as long as possible!