Is your husband not right when he says he isn't enough? Otherwise why would you pursue another relationship?
I'm asking these questions to hopefully help you to explain it to your husband.
Why do you want to expand your relationship to include this other man?
Be honest with yourself and your husband will likely hear it in your voice. Are you doing this because it is something that interests you only if you are "allowed" to do it, or is it a need of yours to share yourself with others intimately?
In other words, why are you poly? Be specific, be clear and then tell him. If it is because you are lacking excitement then tell him. If it is because you believe in a philosophy of intimate openess then tell him.
A lot of confusion and misunderstandning in these situations seems to come from people not being coherent in why and what they want. If we honestly answer those questions for ourselves then it will likely be much easier to communicate them to a partner.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over