Does he think people meet people and start out with something like, "Hi, I'm Emma. I've been assaulted."? Or, "Haven't seen you around for ages, how have you been?" "Well, I was raped by my ex, and I couldn't face going out, because I didn't know if there was anyone else I could trust, if someone I loved could do that," Most likely you'll get something like, "So-so, I guess," and a change of subject, or possibly, "Not so good, but things are getting better now."
We're just now socialised to talk about having a bad time. Most of us, when asked, "How are you?" will automatically say, "oh fine, thanks," regardless of whether we are. And to talk about assault or rape - it just doesn't happen easily. Even if you're with someone you really do trust, it doesn't happen easily, if at all. I bet no blokes he knows have ever told him if they've ever been sexually assaulted or assaulted, unless it was in some situation where they could appear heroic in some way, e.g. "I fought off the 3 guys who appeared out of the alley."
I only know two of my friends who have been assaulted. I am willing to bet that more of them have been, because just not telling me doesn't mean it can't have happened.