I was just about to post about this! DS also hates his carseat. Yesterday we had to go on a long trip. He is not one that falls asleep in the car either. After I ran out of distractions, he cried and fought it for 1 1/2 h. He fell asleep for the last 10 minutes of the ride home. I know it was alot to expect of him, it was a VEERRRY long day.
Does he suffer from reflux? My DS did, and when he was younger he hated the carseat in part for that reason. That particular position seemed to make it worse because he was hunched over.
He got hot and sweaty and I think that is what made it so difficult for him. My niece also hated her carseat. He is 6 mo so I sang songs, brought toys, played games, but it didn't help for very long. I wish I could give you more suggestions, just wanted to tell you that it is the same for us, too.
Hugs