My husband hated the word "metamour." It sounds like your husband prefers a DADT model. Your husband might never understand. You have two different views of relationship structures, and they are conflicting.
You have to realise this is a massive change for him, and this is not what he signed up for. Yes, people change, and change is a beautiful thing. With this new territory, there are things that he has to be okay with now that he never envisioned. You being gone for days or weeks with these other men, sleeping alone, comforting you if you are upset behind one of them (my friend calls it being an emotional tampon), possibly sharing holidays or birthday celebrations with them, you being intimate with them, and if you decide to be out, that means he will be outed as being part of a poly marriage. People can be negative and judgemental of the mono partners in these situations, and it is unfortunate. None of these things were part of what he saw in his future with you. He is probably feeling uncertain, inadequate, and wondering when or if he will get replaced and pushed out of your heart. It is a scary place to be. He is grieving for the loss of what your marriage used to be, and it is like a death. It is the death of something old and the birth of something new and possibly unwanted by him: a poly marriage. Just like when a loved one passes, he has to go through the stages of grief. Giving him time is your best bet.
It is going to take awhile for him to "get his bearings" per se. I would suggest seeking a poly-friendly therapist. They are not the easiest to find, but if you find a decent one, who can see things from both sides of the spectrum, it might help with this transition. I would encourage your husband to join here, too. Maybe he can ask questions that he is afraid to ask you or just have a place to seek support
YouAreHere has a really good blog Her Blog
. Her boyfriend is poly, and she is mono. She details some of the things that cross her mind and some of the issues surrounding being the mono partner. It makes for an excellent read, and it goes to show that mono partners and spouses have their high days and low days.
I hope you start a blog, too. It can be cathartic to write out whatever you are feeling.