Lu, it is killing me! I can't stop him!! My husband keeps joking that "he's got music in his soul" but it is SO INAPPROPRIATE! and he LOVES it.... we are trying to expand his repertoire!!
Also, trying to figure out if he is gender confused at all.... maybe he wants to be a girl?? I cannot understand his fascination with these performers.
I agree with kathleen'smum that I'd nurture in general the interest he's showing, since at least part of this current obsession must come from a genuine love of music.
I have no idea about gender confusion, but I don't think it's necessarily that. Maybe he's metrosexual.

This particular direction might just be because he likes music, but other little boys he's around don't talk much about music right now, while the girls do. Does he have little girls who are good friends right now? When I was young, the only kid in my neighborhood who was on my wavelength was a girl and we became best friends for a long time, although (thank goodness) there was no Justin Bieber to speak of back then, and she was sort of a tomboy to boot.