I suspect the feelings stem from the world changing around you without warning. You were in a mono relationship and then your partner shows up with an interest in poly AND somebody else in whom he's interested at the same time. There was no ramping up to a change in the relationship. Toss in the stress from the way he's conducting himself regarding this other relationship and you've got a whole lot going on.
AS far as how he's handling himself goes, I'd recommend NOT taking a hands off approach. An essential part of polyamory involves everybody knowing what is happening--and he's hiding that from her. Another essential part of poly life is doing right by existing relationships when adding new ones--and if he's not forthcoming with you, he's not doing right by your relationship.
As for letting him learn from his mistakes? The consequences for bad behavior are broken relationships. Do you really want to let him learn in that fashion, particularly since one of the relationships he can break is yours?
I have to say that I find his interest in poly to be insincere, based on what you've said. If he has read "Opening Up," he'd be aware that one has to be open and honest in poly relationships. He's obviously not doing that. That suggests to me that he's just wanting permission to "cheat" and isn't serious about building open, honest relationships.
I reckon your anger is justified. I suspect you're picking up on subconscious clues that he's not serious about poly relationships and is exploring the world of cheating or trying to find out if he wants to leave you for someone else.
Now, take what I just said with a grain or two of salt. All I have to go on is what you've posted, so I'm missing out on far too much information to claim any great insight.