You have already been given some great advice in what has to be an extremely difficult and rather painful situation - losing the trust of the person who promised themselves to you is a big blow on so many levels. Rebuilding that takes a lot of work. Unfortunately entering into poly while rebuilding this makes it even more difficult - I know, because I did it.
One other idea that I would like to run past you, to see if it finds resonance...
You have been monogamous, and your husband feels that he is poly and is suggesting that both of you become poly (in other words, you find other folks to love as well as him). There is another option (I know, because I did it
) - it's usually referred to as mono/poly - one partner is poly and has multiple partners, the other chooses to be monogamous and have only one partner, but it perfectly happy for their partner to have others in their life.
I guess I don't want you to feel forced into having multiple relationships if that's really not what you want. Mono/poly is perfectly viable as a relationship style - I know many who do it. It's really all about finding out what you want, and not being afraid to structure your life around your own goals and desires.
Hope this makes sense.