Get a birthday button either from your resort or from guest relations at any park.
A neat place to go for a birthday is Coral Reef. You can call the restaurant and have a diver swim down holding a birthday sign. You might even luck out and Mickey may be scuba diving too. You can also order a cake (in advance). If you are staying on property make sure the resort knows what day his birthday is then he is likely to have balloons and a card waiting for him when he returns from the parks that day. My son had one from Buzz...it was really neat for him!