Although my child doesn't have a PWS (she has CMTC on the entire top side of her right leg) I can understand your feelings. I remember 9 months ago (yesterday) when my daughter was born I had the same feelings. At this point those feelings are long gone. They completely went away when she started having a great number of other medical issues that may or may not be related to CMTC. What I am trying to say is that just because your child has something that makes them look just a little different that isn't such a big deal when you consider what could be.
Your son will do great (and from other parents that I have talked to they say boys usually do better). You will realize as the time goes by you don't notice it. I see Becca's leg when I change her diaper but it isn't like those first couple of months when it was all that I saw. I remember keeping her leg covered with a blanket when we went places so nobody would see it. The other night I went to a kid carnival in our town. I knew most people already know about it (small town and all) and she had capris on. Nobody said anything and I didn't really notice anybody looking at it or saying anything (not even little kids!). I have even purchased some shorts for Becca. I don't know if she'll wear them a lot since she doesn't regulate her body temperature very well (her normal body temperature is very low) but I won't hesitate to put her in them because of her leg. If you are comfortable and open with your son's PWS you will find the others are accepting too! Take care and we will be thinking about you through this journey you are beginning.