As PP said, wear the button and/or make sure you place it on the table when you eat. At some of the character meals, you may even get a signed birthday card! (At Tusker House, my son got a card signed by all the characters at the breakfast. It was an unexpected, and an awesome, suprise!)
You can also order birthday cakes at most restaurants ahead of time. If there is one particular day you are celebrating his birthday on, you might consider having a birthday cake in his favorite character. (They can also deliver to most resorts). Search for the 'Cake Thread' for some wonderful pictures.
You can arrange for something special to be delivered to your room by the Disney Florist.
Be sure to tell the front desk at your Disney resort that you are celebrating a birthday and ask for a call from Mickey and Minnie. The call will come on your room voice mail and you can listen to it over and over (as long as you don't delete it that is!)
Have fun!