My daughter was the same way. She would cry so hard it sounded like someone was torturing her - I had to pull over to calm her down.
I bought one of those clip-on toy arches for strollers. It has little stuffed toys dangling from it - some play music when batted (she loved that). I clipped it on to her car seat. Another thing that helped was one of those music-playing things for the car that light up. The one that has a remote control for the driver is the best (make sure it's loud enough to be heard over car sounds - one with adjustable volumes is best just in case he falls asleep but you don't want to take any chances by turning it off).
It didn't totally eliminate the crying but the crying wasn't as bad. She would start to cry and I would talk to her in a soothing voice and tell her that she just needed to sit a little longer. Sometimes I would sing a song with her name. Her name is Kiana so I altered the B-I-N-G-O song and Oscar Myer commercial song (desperate) with her name. There was a girl whose name was Kiana and Kiana was her name-o...you get the idea. My Kiana has a first name, it's K-i-a-n-a, my Kiana has a last name...I love to play and feed her every day...(just get creative). Believe it or not that really helped. The remote song thingy was useful if the singing wasn't helping.
The toy bar entertained her and I would change it up a bit by attaching some of those Lamaze toys to it every other week or so. I tried not to pull over at every whimper (unless she was in hysterics) so she wouldn't get used to my pulling over every time she cried. After a few weeks, she stopped crying. Now she's 6 months and doesn't seem to care too much about the toy bar. Her favorite thing is one of those Whoozits that she loves to talk to, chew on, and swing around while she's in her car seat. I also bought a Fisher Price window screen with a bright colored teddy bear after the Babies R Us one dissentgrated. I found her watching the teddy bear and in awe over the colors.
Really I think that they get upset because they can't see us. Talking helped sometimes but other times it made it worse because she could hear me and wanted me to pick her up. Another thing that I did was to try to travel after she had a bottle and was a little sleepy. This way she just slept the entire car trip. You just have to play it by ear. Don't worry - he'll learn to tolerate the car trips after a while!