S.G.
I have been on many roadtrips with my kiddos but never one this long. My only advise is to stop and let him run around every 4-6 hours. There are parks everywhere and if you have GPS you should be able to find one wherever you are.
My kids also like when I sit in the back with them. I don't sit with them the whole time, just when they are really getting bugged. This way it is a special thing. I would also take lots of things to do but only give him one at a time.
Good luck and I hope it is a fun trip!