Hi Sarah,
While the weather will be hot during your trip, you've chosen an excellent time to visit Walt Disney World. Your travel dates will cooincide with some of the lowest crowd times at the parks. What an excellent way to honor your son's birthday.
One of the easiest things you can do is get him a free "Birthday"
button at any guest services office in the parks. (These may also be
available from your hotel front desk.) This will alert cast members and
other guests that your son is celebrating. There's nothing
guaranteed, but sometimes he'll receive small perks.
A great way to honor your son would be to get him a special gift from the
Disney Floral and Gift Basket Department.
They have wonderful gifts of food and souvenirs at all
price points that could be placed in your room as a surprise.
Another fun idea is to surprise him with a cake at one of your table
service meals. You can arrange this via the Cake Hotline at
407-827-2253. (I know, a cake hotline, how cool is that!)
For more tips on celebrating, you may want to head over to the
independent website blog.touringplans.com and search there for an
article called "Trip Planning 101: Celebrating at Walt Disney World."
I hope you all have a magical vacation!