I can't really guess, but I know that I'm finding for myself that my intellectual thoughts on a matter and my emotional, visceral reactions can be quite different.
For example, I am intellectually all on board with nonmonogamy. I have been for 16 years. But sometimes I get emotionally f-ed up by situations and it takes me by surprise. And usually-- for me-- it's because ultimately deep down I have a lot of feelings of being deficient, and because of that I don't feel all that secure in ANY relationship I'm in, so adding in another person just pushes all of my "I'm not good enough" buttons.
Maybe, instead of working on talking about poly-- he needs to do some therapy for himself and investigate his self-esteem, self-worth, and feelings about being "good enough". Because in the long run, he is feeling deficient, then he will always *feel* like you are doing this because he's not enough-- even if intellectually he understands that's not the case.
And I recommend Radical Acceptance if he wants to investigate the possibility of this being the case without therapy.
Or it could be something completely different.