Could keep it simple when you request a moment of his time.
"Could you be willing and able to have a conversation with me about polyamory and if that could be something you and I explore or not? I'm not saying we go off to practice polyamory. I saying... could you be willing and able to TALK and have that sort of conversation with me? I would like to know where you stand on that."
Then see if he is willing and able.
And if not willing or able -- ask if he could explain what blocks him from having intimate conversation with his partner -- you.
Because his willingness to TALK about (cookies, golf, polyamory) is one thing.
His willingness to (bake, hit balls with a club, "do" poly) is another.