He probably has what he feels is a good reason not to talk about it, and I agree, pushing the issue may push him away. My concern goes a little deeper, and seems to address many of the problems I see in the relationships around me. First of all, the past may be in the past, but our past is what makes us who we are now. Hiding from it is incredibly unwise! Learning from it, however (as opposed to dwelling on it as some tend to do) is the way to go. I have found that the best time to have a candid discussion about such things is near the beginning of a relationship. If it causes problems, then you see right off the bat what you have to deal with, relationally speaking, in the other person as well as yourself. If one or either is unwilling to share, then the reason for that unwillingness, I believe, would then be the discussion.
And I'm not talking heavy-duty, ground-breaking "We need to talk" kind of discussions here. I'm talking exchange of information and a reasonable expression of reactions. It's sad, however, that so many people simply can't handle the truth, be it their partner's or their own. I firmly believe this is very close to, if not the true basis of much marital strife in the world.
I approached things much differently after I pulled myself together after my first disastrous marriage, and my relationship with my husband of now 7 years is so much better than anyone else's I know (and those people happily admit it!), and I know it's because we are both strong and we aren't afraid of hurting the other: we have both been taught by our pasts. We share not only our previous experiences, but we are able to talk about the most difficult things, because we love each other more than we want to hold on to our pain and bitterness (we love each other more than we need to be right!). But I knew I couldn't have much to do with him if he couldn't handle what it is that makes me me.
Because of our openness, my husband has been able to help me heal the pain and scars of my "colorful" past, as I have been able to do the same for him.
It's a 2-way street, though. If one is unwilling to share, for whatever reason, they most likely require time and trust (both being in short supply for most of us women!), and the strength to not fly off the handle or lash out about something that happened before you were even in the picture.