I wish I had noticed this thread earlier. DIP Jr is 9 and loves to read (a trait fortuneately inherited from my ex). Right now it's Prince Caspian obviously. So I think he would love the Phantom Tollbooth.
He also love Jules Vern and HG Wells and rectly read 20,000 Leagues Under The Sea and War Of The Worlds. He also likes the Goosebumps series.