I am sooooooooooooo sorry!
There are no two ways around he, he IS a cheater. It sounds like he's using poly as a cover so he can have his cake and eat it too without doing the work needed to keep things open and above board.
Trust is one of the cornerstones of a good relationship, mono or poly. If you don't have trust there really isn't much there. Trust can be regained IF the one who's not trusted wants to put the work into being trusted again. Does he truly want to put that work in so you will trust him again? I mean REALLY work at it, not just saying the words to ease your mind.
Got to get ready for work. You know where to find me if you need me.
There are as many ways to do polyamory as there are people practicing it!