Lots of options for celebrating a birthday!!
They have the Pat E. Cake Birthday Party at the Plaza Inn, where you get your own personal cake to decorate & a drink. Mickey & Minnie come in at the end for photo ops. It is $15.99 per person. You can book that thru Disney Dining 714-781-3463 or set it up at City Hall if you decide to add that. It is held at 2pm daily with weekends adding another one at 4:15pm.
You could do a character meal, there are 5 to chose from:
Goofy's Kitchen does breakfast/lunch buffet or a dinner buffet. Chef Goofy, Pluto, Max & usually a princess & at least one or two more characters frequent this meal. (we've seen the Mad Hatter, Geppetto, Baloo, Brer Fox, Jasmine, Aladdin, Chip, Dale, Max, Pluto & of course Goofy)
Ariel's Grotto has breakfast, lunch & dinner, but this is the Princess Meal & your son might think that is gross!!
Storyteller's Cafe has a small breakfast buffet or order off menu where the characters tend to be the animal & jungle characters.
Lilo-n-Stitch has a breakfast buffet or order off menu. This is a tropical theme.
Minnie & Friends is held at the Plaza Inn & is a breakfast buffet. A lot of the classic characters are at this meal. We've seen Minnie, Fairy Godmother, Pooh, Eeyore, Hook, Chip, Dale, Max & Rafiki.
I highly suggest making PS thru Disney Dining for these.
The other option is at many of the restaurants in the parks or hotels, you can let them know you're celebrating a birthday & get the Disney Pirate Treasure Chest which is a single serving decorated cake, compass magnet, pirate Mickey flag inside a treasure chest. These are $15.00 or you could arrange to have a cake made thru Disney Dining. The price for a 6" cake is approx. $25.00
Make sure your son gets his birthday button from City Hall. He will be wished a happy birthday many many times with this on.
I hope you have the best trip & your son enjoys his birthday!!!