W.S.
I am a mom of 2 boys, these are just some suggestions: Try talking to him. Even if he is screaming, just keep talking in a low toned voice, like look out the window at what ever is out there. Turn the music "up"! My son's LOVED the music up. Not super loud, but enough where he can't hear himself cry. Maybe find music "he" likes. or make sure he is not hungry, diaper changed, doesn't have gas, give him a comforting toy ie, blankie, pacifier,stuffed animal. Put a fun and calming sun blocker on the windows. Hang some toys around him so he can watch them or play. What about a DVD player and put some Sesame Street in there? My youngest used to scream in the car, but we just kept on trying different things and not long after he was ok with it. I hope you find what works.
W.