I've been doing what was an 8 hour trip at least a few times a year since before the kids.
With kids its usually around 10 hours, but has been 12! Not easy with 3 on your own.

Best investment yet was the protable DVD player. Wiggles & Hi 5 are great for them at that age. Well thats all DS watches. Go early morning. Even 4am. He'll likely sleep a bit more than usual being in the car, but maybe not. I can't travel at night at all coz DS can't see, so just gets bored, & scared & cries non stop.

Toys that make noise are good. Lollipops...sticky, but quiet! crayons & paper. He should be able to have a go at them by then.

Make sure you have a spare change of clothes, plastic bag & maybe even spare car seat cover on hand...car sickness is not pretty without them! (Been there a few times, usually unprepared since my kids can be pretty random with it. Fine for 6 trips, then... Always ready now!) Even just a sheet or something over the seat. Easier to pull that off than try to clean out the seat on the side of the road.