Speaking as a parent of a son that has had troubles at school from the usual bullying (nothing to do with his family life) and who has not had the happiest life from his parents both having had troubles (not been an easy 27 years eh), the thing is, my son, is also the one that is the most accepting of me all the same.
On my 50th birthday, he made me a crying mess when he gave me a birthday message he had typed out that was utterly accepting. I have never experienced a better moment in my life. I have told him, he will never want while I am around as a result. He can do no wrong as I see it.
That, and you might want to consider, your son, might know a great deal more than you think he does eh. He might already know all there is to know. 15 doesn't mean stupid

I think hiding this sort of thing from kids, is more about the adults having a problem with it than it has to do with a child having a problem with it. So 'protecting' him for the next 3 years, might really just be no better than insulting him in the process, just writing off his ability to understand.
If you love your son for real, and if he loves you for real, your gender isn't going to change any of that. You might be surprised eh.
Kids say the funniest things. He might well ask, 'so if you become a woman, then does mom get to do you half the time?'.