I am sure you will be pleasantly surprised at how well he does without tons and tons of stuff. We just recently went on a trip up to Iowa and drove straight through there and back with my son who is almost 3 and at the time 8 week old daughter.
We took the portable DVD, but he only used it twice! We checked out some books and children music cd's at the library and those were a big hit. We took the coloring books, but that was a one hit wonder. Maybe you could buy one of the mini magnadoodles if your son likes to draw...that would eliminate all those crayons getting dropped.
Pack lots of fun and different snacks and be sure to stop and let him RUN RUN RUN. He'll probably doze off a lot and before you know it, you'll be at your destination!
Have fun!!