Agree with Panda, in not putting him on the spot.

I also pose this other question: Do you really want to be the girl that broke them up if that's what happens? Even if it were what he would want eventually, you'll forever be in the position of being the one that caused the breakup.

So if it is to be, if he really likes you, let it play out and just be a friend. NOTHING MORE. Until he's unattached, he's out of bounds, IMO.