WL has a 'newspaper' with info on the resort that you're given on check-in. Last May, I believe it had info on ordering cakes, and I think it requested that you order it in advance -- perhaps a day. (So, if his b'day is at the beginning of your stay, you might want to contact WL dining ahead of time.)
We've been there several times to celebrate my husband's b'day, and twice we've found a complimentary cake in the room, a few hours after arrival. Both times, we mentioned the occasion when we checked in. Of course, if you want to guarantee a cake, you need to order it. The cake we got was very small (~ 5-6"), but sinfully, decadently, rich chocolate and the two of us got many pieces from it. If that's one of your choices, don't be disappointed when you see how small it is.
Hope you have a great trip!