Have you explained polyamory in abstract terms, or connected with something that you are? If you have told him that you are poly but you feel he doesn't "get it" yet, then that's a very different conversation from what you would need if you had only talked about it in the abstract.
How long have the two of you been seeing each other?
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