Your DVD should be a lifesaver for the trip, make sure those batteries are charged!
Do you have a travel potty, like a "potette plus" from babies r us? You can set it up in the car when you stop, so that your kid doesn't have to use the gross rest area stops.
When you do stop at a rest area, hopefully there will be a nice grassy area for him to run around a bit and get his wiggles out.
Bummer that he wont' draw in the car, maybe pick up some small treats from the Target $1 section or party favor section and wrap them in tissue paper, so that he has little things to play with if he gets bored of DVDs.
Do you have any of those annoying kid toys that make music and sing songs? They'll drive you nuts but will probably keep him entertained. (like the leapfrog alphabet pal, & stuff like that)
Good luck on your road trip!