Hi Amanda!
It sounds like your son is on his way to being like my 2 sons - they love dressing up as their favorite characters and playing constantly!
We have accumulated quite a few Disney costumes for them over the years, as you can imagine! There are so many to choose from, and they are available in many different retail locations throughout the Parks, Resorts, and at Downtown Disney & Marketplace. You can also check online at the Disney Store to order there, or just get some ideas as to what you might find in Walt Disney World.
You will find costumes such as Pirates of the Caribbean, Buzz Lightyear, Woody, Peter Pan, Captain Hook, Mickey Mouse, and more! And now, with the addition of the Marvel characters, you will find all of the Avengers costumes, such as Captain America, the Hulk, Spiderman, Iron Man, & Thor!
The Pirates League, which is the pirate makeover attraction, is for children ages 3 and up. I am sorry that he is not quite old enough to experience this, but maybe next time?
I am sure you will find something wonderful that will be perfect for your son!
Have a wonderful time at Walt Disney World!