We travel a lot.
Bring interactive toys that he likes - like the Skwoosh ball if you have one, or if he's interested in shape-sorting boxes and you have one of those, etc.
We have snacks, toys, books all handy and rotate through them. I usually sit in the back because it's easier to hand toys around, give snacks, etc. And we figured out early on how to nurse while baby and I were still buckled in (not in town, on the highway). That meant that if baby needed to nurse to nap, that could still happen. And it meant that when we stopped, instead of nursing and THEN running/crawling around, we could just go straight to the diaper change/playing.
We stop every couple hours for about 20 minutes. If you have a Garmin or etc., you can 'search' for parks in upcoming towns so you know where to stop and play .... if it's rainy/snowy, then stopping at a Target or mall are usually are our alternate stops. Grocery stores also work, but not as well.
I really think that the fact that we stop frequently is the reason that our girls love to travel. I know a lot of people try to just floor it and reach their destination, but if kids are used to knowing that they can get out and move around, then they will enjoy travel more. We've traveled from northern WY to FL, all through the Southern US, several times to the Pacific NW, out to WI etc. Lots of time in the vehicle (my four year old used to call it, "Home," for awhile
Bring a ball to roll around outside, maybe, too?
And make sure you've got accessible wipes, etc. for while you're in the vehicle.
Also, if carsickness becomes an issue - fold a square of newspaper and place between your child's tummy and shirt. For some reason, it almost always stops carsickness. My eldest had terrible carsickness when she was little; an MDC mother told me this trick when I posted asking for tips, and even though it sounds bizarre, it really does work.
Good luck, you'll do fine!