Keep everything as normal as possible, meal times, potty breaks, etc. Include him in conversations, don't expect to sit up front and chat and him be quiet in the back seat alone, sit with him too. Etch a Sketch, a White Board and erasable markers along with a sheet or blanket to put over his lap in case he drops a marker. Big manipulatives like toddler sized Leggos, soft books with pictures, games with mom or dad like pointing out the window and naming trees, buildings, or look for a red car, a dump truck etc.
Steven has traveled with me since he was 6 weeks old going to archeological sites and on vacations. He loved having his own battery operated tape/cd player to hear his own music too.
The biggest and most important piece of advice I have though is to make sure he is included in your conversations and interactions so he doesn't get bored. You'll know when to leave him alone, he'll zone out or flat out ignore you.
Have fun, I loved road trips with Steven.