Cut off the relationship right away. All you are doing is letting him have his cake and eat it too. He is still attached to his wife and unless the day comes when he is divorced from her entirely, you will never have a satisfying relationship with this man. It will be tough to break it off completely given you care about him and you work with him, but in the long run you will be so much better off if you do. Keep in mind that you probably like him even more because he is unavailable.