Make sure that the b-day is noted on your ADR so that CM is aware and mention it again at the podium while your son is not listening just to be extra sure.
You can make sure that the hotel is aware as well. Sometimes they will place cards in your room from particular characters.
When you go the parks, before entry, stop at guest relations and ask for a birthday pin for him to wear. Different CMs and characters wandering the park may surprise you with something or just birthday wishes.
You can make arrangements for a cake if you want that. I'm not sure how, but I'm guessing you can call WDW dining and find out.
You can also make arrangements with Disney's florist for room delivery of a birthday surprise, or more economically, bring something that you prepurchase from Disney shopping or outlet for him to find on his b-day morning.
Just a few thoughts...Hope it helps! Have a wonderful trip.