Found out something really bad about the guy I'm seeing..help?

So I've been seeing this guy for a while (nothing too serious, mostly just hanging out and well ya know), and he is absolutely wonderful and I really care for him, and he's just so darn cute, and we are both in a point in our lives that relationships just aren't for us, so we have a great understanding. But I found out from his best friend yesterday that he has physically abused every one of his past girlfriends. I would have never ever expected that from him...never. He has never been violent with me what so ever, not even an argument. So needless to say, I was shocked to hear this news. So, I know I have to get out of this and get out fast, but the best friend made me sware i wouldn't tell my guy what I had heard and who I heard it from. Sooo, how do I break it off with him?? He knows I have no reason to. We spent the whole day together Saturday and told each other how much we liked each other...so what do I do?? He's gonna know I'm lying either way! HELP!!?