A few things ;
1- Announcing ones self as 'poly' is never a excuse for previous, or ongoing shitty behavior. Its up to you, the woman who knows him, to decide if thats a cop-out, or if he really was a putz because he was 'hiding' himself.
@. - This is a question. With how he has treated you, and your anger and resentment because of it,..Do you really want to be 'supportive' of him doing this to another person ?
Or, would you rather be 'supportive' by telling him to learn new ways of being, make himself the best person he can be, and then look at the idea of 'dating' ,..LATER ON ?
Which do you think is the right choice ?
One thing about being 'poly',....We are poly, in thought, regardless if we get to act on it or not.
People who want to go out and bump uglies, and worry about the ramifications later, are not practicing polyamory.
Its such a core part of yourself, (when you are actually polyamorous) that you want to be the best person you can be to the people around you.
You want them to enjoy your company, as much as you enjoy theirs.
If he wants to become one of those stereotypical polygamists,..well, thats a whole `nother thing.
Anyhow, just something to chew on. Ariakas has some good points as well. Think long, think hard. Don`t be scared of your own version of the truth.