We were at WDW for our daughter's 5th birthday and she had a blast. She received a fair amount of attention while wering her birthday pin. Lots of happy birthday wishes, a free pin from a CM, things like that.
We also signed her up for a pirate cruise (it was about $30 and she loved it. We did the one that departs from the Grand Floridian, but there are others too). She LOVED this by the way. And, of course, we scheduled a character meal for her birthday dinner. You can arrange for a cake with your dinner, if you wish, but there is a decent chance he'd get a birthday cupcake for free if you don't do buy a cake. Cake prices and selection can vary, depending on where you are dining. Most/all places have a very standard little cake, some places (like the Grand Floridian) have a wide selection of cake sizes, designs available.
I actually prepared a little gift from Mickey and I am glad I did, as she did not receive one from the resort. I just bought a few little things from the gift shop (a ball for the pool, a Pluto pen, a jibbitz for her
crocs) and signed a Mickey post card from him.
No matter, what he'll have a great day; he will be at WDW afterall!