Hi Steven!
There are so many great choices to celebrate your son's birthday at Walt Disney World!
First, I would refer you to this website:
Customize Your Celebration: Birthdays. Here you will find some fun ways to make sure he's "surprised", such as ordering special t-shirts, hats, cakes, gift cards, dining experiences, and even an In-Room Celebration to get things started!
Also, make sure you tell the Front Desk Cast Members when you check into your Disney Resort that you are celebrating your son's birthday. They will usually have a special "Birthday Pin" for him to wear throughout their visit. This way, many Cast Members and others will be sure to wish him a special day wherever he goes. And, if you're lucky, you may find that he'll be the recipient of some extra special wishes being granted from time to time! It's been known to happen, though it's not "guaranteed".
There are some great activities you could choose from if you'd like to really give him a great adventure! Check out
Disney's Tours and Special Experiences webpage to find things such as the "Wild Africa Trek", and other special tours and adventures!
Finally, there are special
Recreation activities he may enjoy, such as golf, biking, fishing, parasailing,and even watercraft rentals! The possibilities seem almost endless!
Best wishes for a magical time at Walt Disney World!