Well to limit the tv time in the car, say "not this trip" and don't turn it on. Sounds simple enough for me. My son doesn't always like it when I say no, but what kid does? Set it so that the dvd player doesn't turn on automatically. It's easier to say NO to turning it on than it is to turn it off after it's already started.
DVDS -- I check them out from the library. My son is 2, so may have different interests/levels than your son. But we like:
- Signing Time (catchy songs, learn sign language)
- Meet the Letters, Meet the Sight Words, Meet the Numbers (made by Preschool Prep Co. They have a few different DVDs). I think they're pretty boring. They basically just repeat the letters again and again. BUT - my son is 2, finds it interesting, and now knows a LOT of his letters and just about all of his numbers. We watch this one in the car fairly often, as we live out in the country and it's a 30 minute drive into town.
We get both of these DVDS at the library for free. When your son is older, Bill Nye the Science Guy has DVDs now (at my library for rental too). He is so much fun to watch.
I have not tried the Baby Can Read. But I can't wait to try the LeapFrog movies as another suggested.
ADDED:
Our favorite educational TV show is "Between the Lions" on pbs. Perhaps they have DVDs of old episodes available. The show teaches phonics and encourages reading library books. First time I saw it I thought it was awful, but it's grown on me. My son LOVES to check out the 'real book' of the one that we saw the Lions read on tv (they animate story books and read them to you and put the words of the book on the screen too). Pretty good show.