I have a son who just turned 14. He cannot stand any discussion of my H period. Not good things, nothing. My 17 yo daughter on the other hand will frequently bring him up, his posts on FB, wonder what is going on his head, etc.
I wouldn't tell your son. If he finds out in the course of things, fine. But let him keep as much of his innocence for as long as he can. No matter how you feel about her. That is his mother. I'm sure if she brings a strange man around and is giddy and happy your 13 yo will figure out exactly what is going on.
