Since you are not married it's not an "open marriage," but an open relationship. You can tell him you are not comfortable with an open relationship and move on from there. Unfortunately, if he has a strong connection going with OW, there may not be much you can do to change the situation. Think about what attracted him to you in the first place, the characteristics you had, what made him want to be with you?
There is no arriving, ever. It is all a continual becoming.
