Aww, that's so cute! Hmm...I'd just talk to him about wishes and how they can come true, and then ask him if he remembers what he wished for at his uncle's birthday. Then say "guess what happened, we ARE going to Disney World!
My idea is kind of lame, but I'm sure someone will come up with something creative.
Oh, an idea just came to me! A postcard from Mickey addressed to your son, "I heard your wish and have arranged for you to come visit me in October..."