EDIT (originally I didn;t think there was a Game Boy version...)
Well, I don't know if this belongs here, either, but, being the game expert I am...
The game Walt Disney World Quest: Magical Racing Tour does have WDW themed race courses. It was released for the Sony Playstation, the (now defunct) Sega Dreamcast (the version I have), and the Game Boy Color (I didn't think it was, but I did some research... I was wrong originally!).
If you found the Game Boy Color version, it would work on the Game Boy Advance SP. However, used Game Boy Color games can be hard to find. Also, I read that the Game Boy version isn't very good. If your grandson has a Playstation or Playstation 2, you could get him the Playstation version. It's a lot better.
Hope this helps!