The problem is that is his decision. For me, it took the emotional equivolent of being run over with a ten ton truck in 2007 to let me know I needed to deal with my abuse, since no it is not an easy decision to make, still more I imagine if a person has hurt someone else due to their abuse.
You might direct him to this site, suggest he reads Mike Lue's victims no longer, or sees a professional, but ultimately you can't change things for him and you also need to considder what you need in your life too, since you can't live your life waiting for someone else who may never want to change, though the fact that he told you is good since it shows he's not being completely closed in about it.
As to why this happens, well one of the hardest things I myself had to realize is that my own abuse had absolutely no reason whatsoever, it was just a joke, something that happened, indeed I don't think any of the people involved would really remember, still less believe the effect it's had on me and my life. Then again, my abuse was very much bullying that escalated into s/xual humiliation and gang rape, so other people might be different.
That however isn't really the important question, indeed for me realizing that wondering "why it happened" was not helpful was something I needed to do myself.
Perhaps you could considder this, think about you and him and what you should be doing carefully.