Our favorites are the Leap Frog videos. Start out with the Letter Factory. Not that just having those videos will guarantee anything, but my oldest son loved the movie and liked to watch it a lot, even after he knew all his letters and sounds. But what shocked me is that his baby brother learned all his letters and sounds by the time he was 19 months, just by watching that video!
Leap Frog has a whole series of videos that my kids like, and they're truly educational, not just hand puppets "dancing" to inane music (sorry, I don't like the Baby Einstein garbage)
My parents live in Missouri, so I've made that trip with babies/toddlers several times. I have an iPod with video, but we only pulled that out when we absolutely had to. Books, songs, and toys were a great way to use the travel time to interact instead of just zoning out the kids. Although believe me, I know that you can't do it for the whole trip!
We have a See 'N Say with animal sounds that plays the Farmer in the Dell and Old McDonald. It's a favorite toy for trips, and we sing along with Old McDonald. Another fave is an activity cube from Big Lots. There are six different hings to play with, and no parts to get lost!
One advantage is that your son is young enough to sleep for a good portion of the trip.
And our favorite thing to take along when our trip will be continuing into the night is a GLOW STICK! Don't get the little skinny ones if your son is prone to putting things in his mouth. My son bit right through one and got the stuff inside (it's non-toxic, but still!) all over his face and blanket. And then he had no glow stick and was sad. Get the big fat ones!
(by the way, Between the Lions and Sid the Science Kid are just now getting interesting to my 4 year old. They were way over his head before, even though he's been reading small words since he was 3. I'd skip on those.)