Now I don't know if I can trust that he won't do something and not tell me because I know he so badly wants to.
That part makes no sense. He didn't have to ask you. He could have been cheating and not bothered to ask! That's all speaking to fear. Take a deep breath, calm down. Do not jump ahead to "what ifs" and make tempest in your mind. Keep it on the actions done/not done.
What has he done? He has told you he wants to explore polyships with you. You do not want that.
The manner in which he told you -- was it disrespectful? Or merely clumsy? Only you are there to know.
- Is he not willing to give up other people? You break up. You do not want to be in polyship. You know that for sure.
- If his manner in bringing all this up is disrespectful and bullying and this is habitual? Break up. Don't even be friends. You do not need a bully for a "friend." Not even if he's willing to give up the polyshipping -- you do not need to be with a bully/abusive person!
- If clumsy -- alright. It happens. But you are better off as friends if he still wants poly and you do not. You want very different things in a romantic relationship. Dial it down to friends then.
- Is he willing to give up polyships? NOT being abusive/bully type? And just wants you to share in his poly books and poly thoughts and allow him poly expression?
- You are not willing? Break up then. It's not fair to keep him from poly expression. A person can start to feel dead that way if their monogamous partner denies them emotional intimacy or only wants to love the "sanitized version" of them. But not ALL of them.
- Are you willing? Then perhaps you can be together still. He closes down to just you. You open up to just books and listening to his thinks and providing him safe emotional space to share emotional intimacy in.