Hmmm... I think a basic overall/shortall pattern would work well for this. If you could alter it at the top so that it would provide more coverage on the upper body, it would be pretty close to what Sora has. In order to make the legs poof out, you may need to use some type of wire or something bendable on the inside that you could adjust to the shape you want.
The jacket could easily be made from a white zip-up hooded sweatshirt.. as for the keyblade, one of my friend's made one EXACTLY like it from wood, so I'd try that. ^^