If you are not afraid of him, consider telling him exactly what you want out of the relationship before you leave him. Of course, if you have already done that, then do what you feel is necessary and right.
But I don't get the sense from your posts that you have really laid out to him exactly what you want. It seems that he tells you what he wants and then he goes and does it and you cope.
But what if you said, I love you, you're a good father, I want to stay in a relationship with you but I need that relationship to be monogamous. If you feel safe with him, then it never hurts to ask for what you really want. You may not get it - and honestly I don't see him wanting to be monogamous from what you've written - but ask for what you want in life. He may not get that you are so close to leaving. Knowing that you are on the verge of leaving may change nothing but unless you ask, you don't know for sure.
Best of luck to you. I think you are doing the best you can in a difficult situation.