Welcome to our forum.
Sounds like you are in quite a quandary, if your current partner is monogamous and won't tolerate non-monogamy, but you want to be non-monogamous and you don't want to leave him.
Basically, if you stay with him, you will not get a good opportunity to be non-monogamous. You'll have to make a choice, one way or another, of what's the most important to you.
While you are deciding, there are a lot of insightful threads you can read here, and you can post any thoughts, concerns, or questions you might have.
Hope we can be of some help to you.