Hello Victoria. Thanks for asking your question here on the Disney Parks Moms Panel! I have a six year old daughter and she just loves dressing up at Walt Disney World. If you have your heart set on a trip to the Bibbidi Bobbidi Boutique, keep calling! I have called for reservations everyday up until my trip before. You could even try heading to the Bibbidi Bobbidi Boutique when they first open to see if they have any walk in spots for that day.
You can find dress up dresses just about everywhere you look at Walt Disney World. Your biggest selection can be found at the World of Disney at Downtown Disney, Castle Couture and the Emporium at Magic Kingdom and Mouse Gear at Epcot. At the gift shops you will find a variety of characters like Aurora, Belle, Cinderella, Ariel, Merida, Tiana, Jasmine, Tinker Bell, Minnie Mouse and several others throughout the Parks. You could also visit your local Disney Store to purchase a dress and bring it with you on vacation.
While I personally haven't noticed princely gear at Walt Disney World, it never hurts to look. I am sure you will be able to find something to accessorize your son with! If he is interested in looking like a pirate, check out
The Pirates League at Magic Kingdom. This experience is a lot of fun for little boys and girls to get all "decked out" like a swashbuckler from the pirate times!
Have a great vacation at Walt Disney World. If I can answer another question for you, feel free to ask!