Each Theme Park and DTD has the birthday buttons at the Guest Relations office. If you're staying on-site, be sure to tell the Front Desk you're celebrating a birthday -- the hotel may do something special as well.
I took DD for her 5th birthday in May 2005 and look forward to doing the same for my son when he turns 5 next year. We had dinner at Chef Mickeys, and she got a cupcake and special placemat from them. (We also had breakfast at CRT, but that might not be your son's cup of tea!) Her "big present" -- besides the trip itself -- was My Disney Girl's Tea Party. Again, not something I plan to do for my son; I have my eye on the Pirates cruise for him.
Le Cellier gave DD lots of attention the night before her birthday; it was a high point of the trip. I may take my son there for his birthday, too.