As to whatever insecurities Matt has about this monogamy thing, I think it's important that someone who's "polyamorous by orientation" doesn't *have* to be poly, any more than someone who's "bi by orientation" *has* to have relations with both genders. It is a *capacity* that one can have -- not a *necessity.*
Maybe he'll never be able to believe that entirely, but as the years go by his trust level should slowly improve. Put very simply, polyamory is not the right way for everyone. It's not necessarily the right way for him, and if you are devoted to him then who says monogamy isn't okay for you too?
He might realize that someday. These things take time.
Love means never having to say, "Put down that meat cleaver!"