It's a tough issue to deal with, for sure. There's no one "cookie-cutter' way through it, either, and there is no guaranteed success, either.
I believe that there are some who are most definitely "wired for mono",and there are others who are mono simply because that is what society has conditioned them to believe. (I know there are others in the poly community that have the "everybody is poly deep-down" theory, but I don't subscribe.)
If the person is wired for mono, and you are wired for poly, then no amount of negotiation, persuading or "convincing" is going to change anything - the two of you are fundamentally incompatible, no matter how much you love each other. In a lot of ways it's almost rude to consider one trying to change the other in order to make it work, because it disrespects the needs.
If the person has been conditioned to be mono, then education may work. I say "may" because overcoming conditioning is hard.
But how do you know which it is? There's the problem. There is no sure-fire way to know. They can read all they can, they can do counseling, but in the end, it comes down to them and what they are willing to do in terms of self-discovery.
I think that the only thing you can do is to make two things obvious to them. First, you love them and want a relationship with them. Second, you are poly, and want to be free to have other relationships. You are willing to work with the person if they want to try to negotiate a way through this, whether it's towards both of you being poly or towards being in a so-called "mono/poly" relationship (which is what I have). But if you strongly believe that you are poly, and you feel that this is non-negotiable for you, then you have to communicate that to your partner, and work through the consequences of that.
I wish you luck.
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb