Do I tell my SO I know his secret?

I've been dating a guy for the past 5 years, (F,M, 29). We've always been on and off, but recently we've been working hard on our relationship agreeing that we both want to settle down with each other, get married, have kids, that whole shit. One of the big issues in our relationship has been our lack of sex life. Absolute lack of sex. It's been frustrating since I never understood why he didn't seem to be attracted to me anymore, but then I recently discovered he's into bdsm. He has no idea that I know about his secret sex life. How do I go about telling him I know? On one hand, I feel like he would be embarrassed and upset about me knowing about it and for that reason I'm nervous to bring it up, but on the other hand, I feel like he hasn't talked to me about it because he feels I can't satisfy him. I have zero experience in it, I honestly don't know much about it, but I'm certainly not against trying to understand it. I know it's an issue I have to deal with soon since I can't settle down with someone that is keeping this huge secret from me while I try to accept the fact that he can't have sex with me or be affectionate with me since I'm not his "type". I never knew this about him and that alone is terrifying since I feel I don't know him, but it's not something that makes me want to leave him either. I need advice from other couples. Did you make it work or was it too hard in the end?