First of all welcome to the group.
The way I see it is that if you can not except your birthmark, how is anybody else going to get the chance to except it? I know you are in that age group and looks are everything, but they are not in the long run. Being that I have a facial PWS I live with the questions pointing and staring everyday since I work in retail. Yes there are some very rude people out there but I only have to see them once and they are gone normally. I guess what I am trying to say is that if you love you guy friend and he loves you then it will not matter to him if you have a birthmark. If he can not see past it then that is his problem not yours. Still to this day I ask my fiancee why I am so lucky to have her and she says she is the lucky one to have me.
One way I guess you could let him know is tell him you have a birthmark before you even consider showing your back. Let him absorb it don't just jump into every detail there is to know about it otherwise you will scare him. Keep it simple and short.
I hope this helps a little. If you have any other question please ask we are here for you.
Talk to you soon.