Are you quite sure that he isn't just mono? It's a perfectly dignified way to be...really, guys...
I know that my hubs and I will have this same problem later on, though. I get it. It's easier said than done to just honor the other person's orientation on the mono/poly spectrum and let each other be ourselves. For instance, despite the wisdom of intending to do that, I know my hubs feels like he should try to push himself to see other people if I am going to do that, and I know that his desire to do so is a little less than genuine. So I hear your side, but it seems like he may be less than willing to be poly himself. (Like I'm thinking my husband might eventually realize about himself.)
So why do you think it needs to be a Z to work? Do you think it would help distract him from his feelings about you dating other people?