Hi Daniel,
The two main theme park differences from Disneyland and Walt Disney World would be
Epcot and
Animal Kingdom. Animal Kingdom is the latest theme park that was created here, and for a 9 year old boy, some very cool rides (for example: "Dinosaur" and "Kilimanjaro Safari"), just to name a few. There are also a few really amazing musical theatrical shows/performances here, that are some of my families favorites. DinoLand U.S.A. is another part which I think would be perfect time spent for a boy this age.
As far as Epcot, you will discover many new experiences including a trip to many pavilions that represent different countries, their cultures, cuisine, movies and some attractions. This is definitely a learning experience done the "fun" way! If he likes marine life, there is also a large aquarium with many facts to be learned here including many beautiful fish such as Sting Rays, Manatees and Seahorses (a few of my favorites!).
There are also 2 Miniature Golf courses here with really neat "themes" and of course 2 huge...and I mean Huge!
Water Parks located at Walt Disney World, which in my opinion are "must see's and do's". All of their resorts....each having it's own unique flair with bonus added features and activities at most of them. These should be visited , if time allows. Put it this way....The Whole of Disneyland can fit into the parking lot at Magic Kingdom in Walt Disney World...so you can just imagine how much more here there is to experience. I hope this gives you a little help and insight!
Have a magical trip!!!!
Brigitta