if i may i'd like to suggest three ideas. they're just suggestions, you don't have to listen to them if you don't want to.
1. boy spies on a girls slumber party and is caught. one of the girls happens to be a witch.. well you get the idea about what happens next.
2. a boy is looking around a toy store and finds some kind of weird doll house. it suddenly does something to him to turn him into a girly doll which a little girl then buys.
3. a boy is accidently zapped into a computer (like in tron) and is somehow put into some kind of a dress. most likeley by someone using the computer to play with him in a paint program or something like that.