You can call the restuarant directly and arrange a cake with them. Tell everyone it is his birthday, and get him a button at guest services at the different parks (they all have different ones). We celebrated my husband's & D5 last week in the park. Everyone noticed the buttons and said Happy Birthday. They even had the barber shop quartet sing to them on Main St. They received birthday proclaimations at Coral Reef, Cinderella breakfast and Whispering Canyons. We also had several birthdays at WC and the whole restaurant sang to them.
Since my son who's birthday is in Dec felt left out, one day became his "Unbirthday" and he worn the buttons and celebrated in Chef Mickey's with the special cupcake. The CM at dining told me when booking,"This is the most magical place on earth, it can be anyone's birthday!" It went a long way to make my son feel included but he is still asking if next time we go, it is in Dec.
Also, call the WL a week or so ahead and let them know you are celebrating a birthday and they will deliver a personally signed card from any character and balloons to your room for free. Remind the front desk the day you arrive. They will not deliver on arrival or departure day.
Good Luck!