I grew up with a different situation with my family structure and my mother lied about it. It was not till I was 18 and after a very troubling time my SIL told me that my parents we never married. I had grown up believing that my parents divorced when I was 3 and that is what I told everyone.

I was not shocked, nor did I have a problem with that fact they were not married, I was just so hurt and humiliated by the lie. My mother had taken my father's name and even wore a wedding ring, but there were things that never added up. She could never explain the lack of wedding photos and would make excuses about why I could not get my full birth certificate, I could only ever have the extract. I understand why she did it, as it was not the "done thing" back then to live in "sin" and have a child out of marriage, but I still felt that she should have explained it to me and then explained why it was not something to be running around telling the world.

I think certainly say something before he becomes a teen. Best of luck with it all.