Varied activities. Spend the time talking about the sights, counting, singing, making rhymes, telling stories, coloring, looking at books, play 'i spy', travel bingo, etc. Don't forget snacks and frequent breaks to walk around and see the sights. Dvds and video games can be helpful in limited doses.
Keep him busy. You will need lots of games, books, toys, etc. To keep him occupied during the trip.