J.S.
Well, my kids are 1,3, and 8. We just went on an 1100 mile trip (each way). The older one had his Nintendo DS which was great but when he was younger he had a Leapster that kept him busy for a long time. There are games that go from preschool on up. We have a Leap Pad too. Look at a second time around store and you can usually find them at a more reasonable price. We also had activity books and toys. All my kids love musical toys. Gets a little annoying for you after while but it's worth it to keep the kids busy! You can play the alphabet game (if he knows his letters) too. You just look for a word that begins with a,b,c....etc. Look on billboards, road signs, semi's, license plates... the kids have fun with that sometimes. We've gone on a 960 mile car ride every other year since they were all 1 year old (this will be the youngest's first time for that - but the 1100 mile ride broke her in! lol). So, mine are pretty much used to it. We try to keep the toys rotating but there are just times you have to ignore them and let them entertain themselves. And naptime is wonderful!!
Good luck. Hope you have an enjoyable trip!