I'm a nearly 24 yr old with a PWS on my forehead and have been in a few relationships (am currently in one now, actually) and NONE of the guys I have been with have been bothered by it--and I have asked! I have also been asked out a few more times by guys I didn't know (one was in a grocery store, actually).
My theory is that if a guy doesn't like you because of something like PWS (or freckles or the colour of your eyes), then who needs him? I know its never that easy once feelings get involved.
So, my advice is to keep your chin up and not worry about it. But also be willing to answer any questions he has if he's curious, or, if you're comfortable, offer to tell him about it in case he's afraid to upset you.
The hope a dream and the wings to be free