You can only "know" so much of someone from his or her past relationships. Alot of times the past is a time were mistakes were made anyway. Things he did or didn't do with an ex, he might do differently with you now. I can definitely understand the curiosity, but why do you want to know? Do you not trust him for some reason? Do you think that he's harboring feelings for an ex?
My boyfriend and I don't generally talk about our past relationships (especially because we've had some dysfunctional ones) at all. There are no details I'd really care to hear about his exes. Am i mildly curious? Maybe. But I love the man he is now. That's all I really care about.
There's nothing wrong with asking, but I don't think it's crucial that you need to know for your relationship to progress. Unless you think he's an axe murderer or something.
“Inside every cynical person, there is a disappointed idealist”--George Carlin