Josee,
Nine is such a great age for a visit to Walt Disney World! So many of the rides and attractions are ideally suited for him (and for you too!).
So each park has so much to do, I love that you are going to make sure to hit all four.
Epcot is fun, both for all the World Showcase venues and stores, but also for rides like
Test Track and
Soarin' Around the World. The
Animal Kingdom Park has the most amazing animals and tours. Make sure you don't miss the
Kilimanjaro Safaris.
But the real question is where to go an extra day, the
Magic Kingdom or
Hollywood Studios? Normally I would easily say to go the extra day to the Magic Kingdom, however if he is really in to Star Wars, then that would be a tie breaker, to be sure. The
Star Wars: Galaxy's Edge area could be worth the return trip.
BOTTOM LINE: Unless he is a Star Wars fanatic, go to the Magic Kingdom. It has more for a 9-year old to do, and the varied lands make for more experiences than one day can contain.
Have a Magical Vacation!