We used to take along a magna-doodle (travel sized) and draw pictures and have the kids guess what it was... kind of like pictionary. They could also draw pictures and have US guess what it was. Take a bag of small books to look through-the kind with "lift the flaps" are always fun for them.
We never did the DVD stuff with our kids, and we did a lot of traveling.
I would suggest that you try to plan to leave early in the morning (he'll go back to sleep pretty easily in the car for at least an hour of travel), or close to nap time. If you are mid-morning traveler (like me)... leaving around 9:00 or 10:00... try to get him up a little earlier, so that he'll be ready for a snack just before you go, then will drop off for a nap right away once you get moving. Let him sleep as long as he will... then when he wakes up try a book or game, then stop for lunch. Let him play in a safe play place (like a chik-fil-a or something.. or if a mall with a play place is nearby, grab a bite from the food court and let him play while you eat). Then back in the car and read a book, play a game, etc... then put on some music and he'll probably take another good nap.
We always tried to consolidate all of our stops (bathroom, gas, food, stretch legs) into one, so that when the kids fell asleep, we didn't need to stop for anything, as that often would wake them. As long as they slept, we kept driving. Just remember to pack whatever "lovey" item he has so he can hold it in the car. It always helped ours to conk out.
(And pack extra baby blankets in an accessible place - you never know when a bout of car sickness can hit them!)
How far are you planning to drive? It will work out, and you'll figure out what works for your little man! Have fun!