Welcome to our forum.
A post I chanced across many moons ago had some really good insights about this kind of dilemma. Here's the addy: http://polyamoryonline.org/smf/index...sg9230#msg9230
The man you're with doesn't yet seem to realize that polyamory is a very serious/important topic for you. You need to think of ways to break it to him gently that, "Honey, this polyamory issue is really important to me."
Sometimes simple, respectful honesty about what's going on with you and your feelings, combined with a humble request for him to consider it, is the most effective approach. But remember, even if he says yes, and you meet a new partner, and get all excited about them, you might get too carried away and constantly tell your original partner how wonderful the new partner is, and not even realize that you're neglecting your new partner. Be careful. (I know I'm talking about something that would happen after he said okay to polyamory, but.)
I hope you'll find the best way to approach him with the subject that will work for you and him. Keep us posted on that.