I really like the Passporter books, my 4 year old loves them also. There's pockets for each day of your trip to keep little paper souvenirs. You can write your favorite memory/funniest memory, etc for each day. There's places for autographs. My daughter & I sit down with it at night on our trips, and jot down all of our favorite things from the day. We fill the pocket with that days little things. The whole book is a great souvenir in & of itself. While the book is not specifically designed for kids, I think kids still enjoy it.
He might like the Hidden Mickey books also. Not a guidebook, but more of a guide to where the hidden mickey's are. I know a few kids who love this!