How should I ask him to Sadies?

Leave pieces of candy in places where he will find them. On each piece, put a clue to the next piece. The last clue should lead to some sort of finale - maybe a conglomeration of candy - that makes it obvious its from you, and asking him to the dance. Find some characters from the game (there ought to have been at least one couple) and put your faces over the characters. I know its probably not as "original" as you may have wanted, but add some of your own personal flavor and it should be a success.