Has he ever talked with you about his hopes/plans for the future? Sounds like he's been hiding things from you, and he's been caught lying to her too. So what might be his pattern? Is it likely that he will change? The why of it all - perhaps there are aspects of his being with you that he wants to continue (otherwise he would leave entirely?). And he wants something from her too. So he carries on with his pursuit of her, and continues on with you behind a façade. Why does he plead with you to stay? Will he stay with you? What are your options? Is there a space where you can go to think this through, and take the time that you need to consider how you would like to proceed?