S.P.
We drive 18 hours twice a year to visit our family, so we spend a lot of time in the car. We have been doing this since our son was 4 months old (he is 4 years old now). As much as my husband and I can stand it, we drive at night, so our son can sleep during a lot of the drive. I highly recommend that strategy if you are able to trade off on driving.
I think the DVD player is a good suggestion. We have a Phillips one that has worked well for us.
For one trip, when my son was your son's age, I wrapped up different toys. Every hour or so (if he was getting restless), I would give him a toy to unwrap. It would take him a while to unwrap the toy, so that would be part of the fun and would occupy him a while. Then, he has something new to play with for a little while. I bought 2-3 really small toys at Target ($3 or less), but I also wrapped up some of his own small toys to play with. Some of the toys I wrapped, I hid them for a little while, so he didn't play with them for a couple of weeks before the trip. Then, it was like a brand new toy to him, even though it was his own.
I hope you enjoy your vacation!