Jonathan! I love that you want to do this for your little guy!
First, visit the
Walt Disney World website to get inspired by the attractions your son is most going to look forward to. When my kids were that age, they would have loved
Toy Story Land. Once you have a list, print out some pages of photos that reflect the list. Make a storybook to read to your son, where the main character is your son, and the end page of the story he goes to Walt Disney World.
Another thing I've done is pack a backpack for my kids with things like sunglasses, sunscreen, a hat, an autograph book (
ShopDisney has some awesome ones), then see if he can figure it out. If he can't, you get to tell him and capture that surprised look.
Our local pizza shop will make specialty shaped pizzas if you ask in advance. How cool would it be to have him open a pizza box with a Mickey Mouse pizza in it, and you can surprise him that way.
Another great idea is balloons coming out of a box when he opens it, or even waiting until you are driving through the main gate entrance and see if he can guess where he is. We've also packed a Disney themed suitcase and let our kids unpack to guess where we're going, sent the kids a letter from a favorite character "inviting" them to visit, and we've purchased specialty MagicBands to give them as a surprise.
I hope you pick the way that is going to be the most amazing for your little guy! Have fun with it, Jonathan.
Anna