Not sure if this helps, but I would like to just use some slightly different terminology to illustrate your relationship.
He is completely sure that he wants to have sex with other people than you. This sounds like something that he absolutely needs, it's a show-stopper or a bottom line to him.
You are completely sure that you want both of you to be monogamous and have no outside sexual partners. You have made this clear that this is something that you absolutely need - it's a show-stopper for you.
If I have got that right, then the two of you are fundamentally incompatible to have a relationship. It doesn't matter whether this started out one way and changed, or was like this the whole time.
Respect each others wishes and boundaries - it sounds like both of you are trying to pressure the other into thinking differently, and both of you are adamant in your positions.
My advice - confirm that these really are non-negotiables for both of you. If they *are* negotiable, then find out what it's going to take to truly make this work. If they are not, then agree that you are not compatible and get on with your lives apart.
On the topic of people changing: There is a book that was the key for me when I was trying to sort out my polyness - it's called "Open Marriage" and is by Nena and George O'Neill - while it's not specifically about poly, a lot of what they talk about is relevant. One of them is the concept of personal growth - in a good relationship we should be encouraging each other to grow as people and to continue developing ourselves into what we want to be. This may mean that we grow in different directions and the relationship may end, but it's far better than trying to constrain your partner to never grow out of fear that one day you may lose them. It is for this reason that a lot of marriages are stagnant and on the rocks.
I am a big fan of personal growth, for myself and for my loved ones. I want everyone to be free to be the best possible person they want to be and can be, bringing themselves to their fullest potential. An interdependent relationship can help to nurture that and give everyone a safe place to be able to grow and explore life. The synergies that a relationship like this offers can often allow *more* personal growth than an individual can have.
"Listen, or your tongue will make you deaf." - Native American Proverb