The phone number for the GF to order special cakes is..407-824-2578. At least that's the number that was given to me. But, please, please, please...unless you are willing to pay extra for something, do not get your heart set on it happening. My dd celebrated her 6th birthday with dinner at CP. I told them when I made the ADR (PS back then!!). I told them again when we checked in at the podium. They did give her a birthday cupcake and a card but nothing special. I didn't expect it so wasn't disappointed. I have heard some people have gotten their tables done up nicely while others have not. I would ask about it when you call...perhaps they can give you some idea on how to best achieve what you want.
I say this only because I would hate for you to be disappointed on your sons bday. My dh made mention of the fact that we were in WDW celebrating my birthday and was upset when no one did anything special..not at the CR where we were staying or for my 'special' birthday dinner at Artist Point. Of course, if he had told me in advance I could have enlightened him!!! Silly ole hubby....but it was so nice that he made the effort.