Well, yes he cheated on you. If he is not open or interested in talking about ethical non-monogamy I would leave the relationship.
I could leave it at that, but sigh... You' know you are monogamous, and it is an UPHILL STRUGGLE to make it work with two new poly minded people who want to be poly. The negotiations and discussions needed to set boundaries and agreements is exhausting when two people ARE on the same page, let alone when one is being less than aware of his own behaviors (yep, that is your bf I'm talking about). If somebody isn't trying to be on the same page as you, you're better off being elsewhere, IMO.
Happiness will never come to those who fail to appreciate what they already have.