We've taken several car trips with our 4 & 6 yr olds.
Post it notes - they come in all shapes and colors and can be stuck to anything - dare him to cover his window, the seat....
lots of new DVDs - Wal-Mart has some great old cartoons cheap
Leapster - on Ebay you could buy a lot of games pretty cheap.
Trip to the dollar store - you can stock up on cheap, cheesy things. I remember buying a pair of glasses w/ cheesy mustache attached and my kids would pull those out when they got bored and would get a good laugh.
We also love to listen to books on CD. I got several from our library.
Buy a bunch of new snacks - stuff you normally would not buy but your child will like. I hate the processed cheese & crackers but bought them for one car trip. Or licorice, that's dessert to me but I let them have a piece here and there if they were behaving.
I always print out maps for each child - just off of google maps - and with a crayon they can trace the road we've driven. Usually I would announce the major city on the map so they could essentially "connect the dots". Example - "ok we've made it to Indianapolis! Trace the road from Chicago to Indianapolis!"
Make sure to bring a football or frisbee for rest areas - you can stop and get some of that energy out. We thought that scooters were a nice idea but it turns out there really aren't many safe places to ride at rest areas. When we drove through the mountains one time we allowed the kids to climb a little rock wall next to the building w/ bathrooms.
Kids love the newness of things so an inexpensive thing to do is put away some of the toys he already has and bring them out in the car - they'll be new all over again.
I would also advise against doing the middle of the night drive. I know it is nice to think that the kids will sleep in the car the whole way. Turns out unless your child is a really heavy sleeper they probably won't. They will continuously wake up and the next day will be hell!
Good luck!!