Try
this site, plenty of activities, printouts, ideas. As does
this site.
This one has links to various sites for activities. At the bottom of
this page, there is a link for activities. And one
last page that sells activity kits.
If your son has a favorite character (Clifford the Big Red Dog; Dora, etc) see if there is a travel kit that includes him/her. When my godson was little (turns 17 tomorrow) i bought him a Clifford travel bag with crayons, activity book, and story. If you can't find a kit already made up, make one up and use that when things get particularly rowdy as a treat. If you can find "read - long" books on CD (or tape) get a couple of his favorites so that you won't neccessarily have to read it to him (I get car sick if I read in the car so LOVE those read along books) and don't forget to get one or two of your favorite books on CD as well (maybe at your library or at a Cracker Barrel - they have a program of some sort) for when he's sleeping! You may also get a longer book on CD that may be new to him that might keep him entertained due to it's novelty.
My nephew particularly loved when I read him a bed time story but loved it more when I made up a story about him (he's 5,...whoa, just turned 6). The story wasn't particularly good but because I used HIS name in it, he loved it. He wanted me to keep going after about 30 minutes of a really, really, really bad story line.
Anyway, have fun and don't forget his "lovey" if he has one! THAT can ruin a trip in no time!