Hi Stacy,
We just returned from celebrating my son's 11th birthday at WDW! It is truly so magical to spend the special day there.
If you are staying at a WDW resort, make sure and let Guest Services know at the resort; oftentimes, a character will call with a birthday greeting. You will also want to get a free Happy Birthday pin from Guest Services at any one of the four parks. Throughout the day, your son will receive birthday wishes by name. And sometimes, little surprises may come his way!
If you'd like to go further, I'd try to eat at a table service restaurant. We ate at the Coral Reef restaurant where I had pre-ordered a custom cake from the cake hotline (407-827-2253) for my son. I also had stopped by the restaurant and requested a scuba diver to swim up to the glass with a happy birthday sign for my son which was an incredible surprise for him.
We also ate at the Whispering Canyon Cafe where the cast member asked all girls under 14 to come up and sing him Happy Birthday. It was an absolutely hysterical moment for all of us!! While I don't think a surprise like that would happen for a 5 year old, the table service restaurants each have a unique plan for celebrating birthdays. Also, general birthday cakes can be ordered without notice when you check in at the podium.
So in a nutshell, make sure and let everyone and anyone know that it is your son's birthday! If you'd like a few additional ideas, we actually have an entire
section within our panel devoted to birthday celebrations.