hey! I had two really great boyfriends once. This experience helped me to understand ways they think and that there IS emotional vulnerability for men as well. My boyfriend, Ryan, was also hesitant about the polyamory thing when we first started. From my experience men often feel threatened when they are forced into a corner to immediately "choose" yes or no by someone they are emotionally tied to. Therefore I would encourage you to create an environment that has the lowest amount of pressure and emotional clutter. This will probably mean making yourself less forceful about your needs and encourage him to think about his. This way he will be able to actually Consider things rationally. I think since you know a few people who are actively polyamorous, then a really good idea is to ask Him and them out for lunch- just to say hello and say that they are happy in their relationship and their needs are not ignored in the relationship.
Clarify with him that you understand that things in poly can go wrong. An example that I can give you: My boyfriend and I were involved with my other boyfriend but things didnt go as planned. I assured him that we would work together to continue looking for common things we both wanted and keep each others needs in mind when choosing others. I think that probably all he is hearing when you bring it up is "wahhhhhh wah wahh I'm not getting what I need and im going to leave you all alone" therefore he is clinging to what he knows and is comfortable with very tightly because he is unsure what else to do. He is probably scared that if he opens up even a little, then you will take things too far, too fast.
He probably wants some assurance that you are willing to take the process slowly and work with him to incorporate his needs as well. I think that you probably feel this is obvious, but sometimes just restating this will open up the conversation. However, although this worked to alleviate Ryan's fears, it did not work for my other boyfriend. He honestly felt that he wanted a more traditional relationship. He chose monogamy, even though he loved me as a person, and I had to respect that. Best wishes for you two!