Being a secret isn't poly, it's cheating, and it's intentionally hurting and dehumanizing someone else. She is a human being, with a right to make her own choices about what is right for her in a relationship. If he is worried he'll lose her, then he already knows that it's unlikely she'd be okay in this relationship style, and is electing to selfishly take that choice from her. Assuming you are sexually active with him and so is she, he is also taking away her right to make decisions about her sexual health.
Would you want someone to do that to you? Treat others are you would want to be treated. And, do you want a partner who is willing to lie and deceive someone he professes to love, who is willing to be so selfish as to take away major life decisions without that person even knowing there is a decision to be made?
There are many forms of poly, and it's very fluid; but, cheating is never poly.