Hi Daniela,
Celebrating a birthday at Walt Disney World is wonderful! You can order a special cake by calling (407) 827-2253. Most of the restaurants offer cakes when you have a reservation there. Customized cakes are available at the bakeries at the Grand Floridian Resort & Spa, the Contemporary Resort, Boardwalk Inn, the Animal Kingdom Lodge, and the Beach & Yacht Club. I would recommend calling the number to see what the options are for you and what size cake you would like. Some fun character dining restaurants are Chef Mickey's at the Contemporary Resort where he can see Mickey Mouse, Donald Duck, Goofy, Minnie Mouse, and Pluto, the Garden Grill at Epcot where he will see Mickey Mouse, Chip and Dale, and Pluto, Tusker House for lunch where he will meet Donald Duck, Daisy Duck, Goofy, and Mickey Mouse, and the Crystal Palace at the Magic Kingdom with Tigger, Eeyore, Pooh, and Piglet.
I know your son's birthday will be just magical, Daniela!