He is monogamous, of a different faith(which promotes monogamy), and he knows that I have a bf.
If you KNOW all that why are you considering offering him a mixed-faith type polyship? And asking him to change a core belief of his about monogamy and go against his faith teachings to boot?
Just different faith paths can be a challenge in a relationship. And here it is different paths AND mismatch in what you want for relationship configurations.
If you want him to know you are poly -- tell him so. That still doesn't mean he's going to want to date you. But you could let him know.
"I find you attractive and would like to date you. I am polyamorous. Would you be willing to entertain talking about what dating a poly person like me would entail? If not it is totally ok to say no, not willing. But I did want you to know you are are an attractive person and I like your style. Paying you a compliment at least brightens your day, right? "