Hi there, Amanda!
Happy birthday - in advance - to your boy!
The only park that's really decorated for Halloween is Magic Kingdom, due to
Mickey's Not-So-Scary Halloween Party that starts in September and runs through November 1.
Almost immediately after these festivities end, the Halloween decor comes down, and Christmas trimming starts to appear;
Mickey's Very Merry Christmas Party begins hot on the heels of Halloween, this year launching on November 9. You are likely to catch the tail end of Halloween or the beginning of Christmas, knowing that by November 8, the Magic Kingdom will be decorated for the holidays. The other Parks and Resort hotels will follow suit thereafter, with the entire place ready by Thanksgiving.
If you are visiting during one of the party cycles, I highly recommend giving one a try. Each one is fantastic, but the "flavor" of each is distinctive to the particular holiday. But in either case, you'll get fabulous fireworks, a special parade, and a much lower crowd experience than you might expect.
Make sure to pickup a complimentary "Happy Birthday" button, available in every Park and when you check in to your Resort.
You can also pre-order a cake for any restaurant reservation that you have -- or order a standard one at the podium of any table-service restaurant at which you dine. To place an order for a fancier cake, just call (407) 827-2253.
If you'd like, send me some of your son's favorites (movies, characters, etc.), and I'll be glad to offer some more specific suggestions. But whatever you do, he's sure to have a fabulous time Walt Disney World!
See you later, celebrator,
-Deborah B.