Make sure it's noted on all your reservations both at the resort and all your PS reservations. I ordered a cake through Pixie Gifts/Beth for my DD's 16th Birthday in October. It was Tinkerbell and she loved it! Some restaurants allow you to purchase a cake. Depending on the restaurant, you can just tell them at the podium when checking in. A few restaurants gave her a cupcake and sang to her. Make sure to pick up a Birthday Badge at Guest Relations at the parks! He will have all sorts of people singing and wishing him HB. My DD was sung to on the bus. She was allowed to be the first to get on the boat from WL. So many people wished her HB that she sometimes didn't know who to thank. Also, a CM in a store in Frontierland asked my DD if Goofy wished her a HB. She replied "no." She picked up the phone and dialed a number and handed the phone to my DD. It was a prerecorded message from Goofy. My DD actually blushed! It was so cool! They have other characters if he has a fav.
Pixie Gifts can also do a gift bag for him. I did this for my DD. I ordered t-shirts, a Tinkerbell cake, and other goodies including Disney Dollars so she could spend whatever way she liked! They will custom whatever you like and I highly recommend them. Since it's his first trip you could do a Guest of Honor Badge, Water Mister Bottle, Disposable Camera, Lanyard, Pins, Etc. There are so many options depending on your budget!
I'm sure he'll have a Birthday you will both always remember!