You have said that his ex-wife is still in love with him, but not what he has done about it. He can't control her feelings any more than you can, and as long as he's not encouraging it or responding to it in any way, then there's nothing you really can or should do. What good is brow-beating him over someone else's feelings going to do? Make it clear to him that you know she still has feelings for him, and so would feel very uncomfortable with them interacting (except as may be required if a child is involved), but otherwise it has no real bearing on your life.
On the other hand, if he is encouraging or responding to it -- then joanspahr has some excellent advice. Step back, and make it very clear to him that he needs to get his commitments straightened out if he wants to make a life with you.