This is part of the process - feelings and thoughts will likely swing widely back and forth between extremes. His thoughts about it seem perfectly natural to me. Our society teaches us that our spouses and committed partners "belong" to us; it is hard to let go of possessiveness. It is such an ingrained "ideal."
I think it is good he is getting it out, and good that he recognizes it's a mindset he has. So many people walk around basically asleep in life, shrug, and say, "I'm just a jealous person!" without any real self-examination. So, as much as it irks you to be thought of as a possession or property, be thankful he is open and sharing these thoughts with you. Don't give in, either of you, to beating yourselves up, or each other. The fact that he expressed what he was grappling with is a blessing, and something to be grateful for, even if what he shared kinda stinks! Even though poly has been on your mind and a desire of yours for a long time, find compassion for what he's going through and don't let frustration piss you off too much.
As John Welwood wrote in his book, Journey of the Heart
, it's absolutely crucial to be fully honest for an intimate relationship to thrive, deepen, and be a source of personal growth. He calls it "applying the microscope of awareness" (great book, btw!). Your husband is actually expanding his capacity for love and intimacy by looking at and expressing to you what is bringing him pain. John Welwood writes:
"Depending on how we relate to love's pain, it can lead in one of two very different directions. If we regard it as a threat, something to avoid at all cost, we will try to patch it over, keep it out of sight. After a while, however, accumulating patches only deadens our sensitivity and our capacity to love freely. Resenting the pain involved in becoming vulnerable to another person causes us to lose heart or harden our heart, and this cuts off the energetic flow between us.
Yet if we can learn to make use of our pain, it can be an invaluable helper and guide on the path. For it exposes and directs our attention to places inside us where we are shut down, contracted, and half-asleep. If I can move with my pain more fluidly, my rigid defenses start to dissolve and I become more permeable to love's awakening influence. And when I can let my partner see my hurt, instead of hiding it away, where it may fester and poison the relationship, this creates greater intimacy between us.
Of course, nobody wants to feel pain. Yet to become a warrior of the heart--one who is willing to risk being wounded in the service of love--we must be able to use the pain that relationship inevitably brings our way."
He is letting you see his pain. He is not whining nor making demands, but being truthful and present and trusting in you that he can share a side of himself that makes him uncomfortable. That is good stuff and can only bring you closer and help build a firm foundation, whether a poly arrangement is in your future or not.
That's why you need to go slowly, do some reading together, discuss everything for a good long while and, at some point when you are both ready, perhaps even find a local poly group to meet others who can share their experiences with you -- before seeking another partner.