There are lots of things you can spend a lot of money on, such as custom cakes, in-room decorations, gift baskets, etc. Not sure what your budget is.
No matter what, tell every single person you come in contact with (reservations Cast Members, hotel front desk, waiters, hostesses, etc.) that you're celebrating your son's birthday. Get him a birthday button - they're free.
There is no guarantee that CM's will do something for a birthday, but my experience is it certainly doesn't hurt to let them know. We've celebrated many birthdays at Disney and CM's have always done something special.