Christy,
Everyone is a kid at Walt Disney World, didn't you know that?
The first, most important thing you have to do is get him a
Birthday Celebration button. You can get it at a Disney resort front desk, or at Guest Services at any of the parks. This will lead to a lot of greetings from cast members, the occasional special treat or experience, and a generally good foot forward.
Next I would consider a
birthday cake at one of your table service meals. Customized ones can be pre-ordered at 407-827-2253, and non-customized ones are available on the spot as well. I think that the latter option is actually better, since you never know when you will be hungry enough to want to add it to the meal, but it makes for a special dinner.
Also, I think most any teenage boy would love to take
surfing lessons. Can you imagine him telling his buddies back home about how he surfed over his vacation? Very cool.
The
Disney Celebrations page offers a few other good options. Personally I like the customized Mickey Ears, but you might also make part of the fun something that is not monetary. Maybe he gets a family coupon that is good for him to get to choose what attraction to go on, whenever he wants? Or he gets one that gives him the power to MAKE you go on a particular attraction!
Write me back with some ideas of things he likes to do at home, as well as where you are staying, and I may have a few other recommendations!
Have a magical vacation!