I'm going to be the one to run the minefield and play devil's advocate on this one, its a situation I'm not unfamiliar with.
Before I start having to Matrix out of the way of coffee mugs, hear me out.
When I first came to the concept of polyamory, I didnt really have the vocabulary (emotionally or verbally) to talk about it. I just knew I had an abundance of emotions that were outside the bounds of what I was used to. My partner at the time was monogamous and not the most emotionally available person in the world. She tried very hard but she had a lot of issues of her own that got in the way.
I handled the introduction of the idea of polyamory into our relationship with stunningly bad judgement largely because I wasn't entirely sure what I was dealing with. It lead to what I would call an emotional affair that came within inches of splitting us up (turns out all it did was light the fuse for demo charges way down the road).
My partner was someone I loved dearly, someone I was very happy with, and someone I tried to be honest with at all times. That didn't stop me derping into a situation that was precipitated by poor choices stemming from a lack of knowledge and that seriously hurt someone I had no intentions of hurting and loved deeply.
First, the obligatory boilerplate:
1. I'm in no way suggesting that his actions were your fault. He's a big boy, he ultimately makes his own choices regardless of the reasoning.
2. Ignorance does not absolve responsibility.
Before jumping to the conclusion that he's a ginormous douche-marble, take the time to figure out if he actually knows what the hell he's doing. I fault someone less for crashing a plane if they had no idea how to fly the thing in the first place.
If this isnt the first time you've heard this kind of talk or dealt with this sort of situation with him, the douchebag factor increases exponentially.
Ellie, you know him best so you are the best qualified individual hear to determine if he's sincerely trying to not to screw things up and just failing at it or deliberately being a biznitch.
I am as direct as a T-Rex with 'roid rage and about as subtle. It isn't intended to cause upset, I just prefer to talk plain. There are plenty of other people here who do the nice, polite thing much better than I can. I'm what you'd call a "problem dinner guest."