Often the easiest solution is crating so he can't see out and get all worked up/frustrated. Crating is also one of the safest ways to ride in the car. Unless you really want to take on the challenging, time-consuming work of de-sensitizing to outside stimulus.
I watched an episode of "It's Me or The Dog" where the owners had the same problem with their dogs. Victoria had them "black out" the windows and put a sheet across behind the front seats so the dogs couldn't see out at all. Once the dogs were quiet on the ride they slowly started pulling open the sheet - but the minute a dog made a noise the sheet would get closed immediately. It worked and they were able to take the black out paper off the windows and the dogs rode quietly.