not everyone is cut out for poly, not everyone is willing to "share" or play nice when another suitor shows up. we men can feel very inadequate, insecure, etc.... we also come wired by nature to provide, protect, defend, keep all the other males away mentality that can not be helped, some of us are more evolved.
the problem you are facing is this. when you marry, you take vows... and people tend to take vows as serious business, as we should. the promise to love, respect, honor, cherish, one another is important in that bond. in my opinion, and this is just my opinion, this is the kind of thing that must be agreed on by both, in order to proceed, into the world of poly, or non-monogamy. you may need to give him more time, but he simply may not be wired in such a way that he will ever be swayed. a marriage is a partnership, a team, a bond, and if this is something he does not agree with, then it will appear to him as a selfish act.
however.... yeah isnt there always a however? if this is something that you as a person feel that you absolutely need to be happy, then you have to evaluate that need, as there will typically be a price associated with it, and not a pleasant one.
please do not let me discourage you from searching for or discovering happiness, my philosophies, and beliefs in life center strongly around the respect, kindness, proper treatment, and adoration of women (ok worship). but i also believe strongly in fair treatment, and balance. i do wish you the best.