Welcome to the party.
These situations usually end in four different ways.
Plan Alpha: Partner gives.
Plan Beta: You give.
Plan Gamma: You both agree.
Plan Delta: You both split.
Alpha: Your partner wants to make you happy and agrees to the new dynamic. They are often unhappy about the situation and dislike it. This leads to general resentment about the situation and general unease for everyone involved
Beta: You agree to remain mono. Doable but if a poly relationship is truly what you need to be happy, eventually you'll start resenting the position you find yourself in. You'll get angry and feel trapped until you cant take it anymore.
Gamma: You both agree to try this new dynamic. This can be the beginning of a new poly relationship and has its own tree of choices and outcomes.
Delta: You both agree that you want different (incompatible) things out of a relationship and it would be best to separate. This sucks the worst in the short-term but is often the best for long-term association if it becomes truly clear that there's no way to make you and him work together.
Alpha and Beta fail spectacularly more often than not. The specifics of all the scenarios differ but the general ideas are near universal.
The choice is yours.
I am as direct as a T-Rex with 'roid rage and about as subtle. It isn't intended to cause upset, I just prefer to talk plain. There are plenty of other people here who do the nice, polite thing much better than I can. I'm what you'd call a "problem dinner guest."