Sorry this has been rough
I will say though, my wife and I have alluded all of this for years, and we had some flirting model that was a great placeholder, but we never wanted to make things too complicated for either of us, plus the place where we lived would not have been conducive to this lifestyle.
It took us years to get to where we are, and even in here, some people think it was a rough way for it to happen, but it was what was needed. Sometimes I wish that we'd have done this years ago, but that would not have made this any easier.
Now, as I'm realizing I might never have been monogamously-minded (I've been reflecting on my old relationships lately), perhaps this was easy for me to hear from my wife, it was the shock that it didn't shock me that startled me. I could not imagine how it would have gone had I probably not been poly all along.
Best of luck. Make sure you openly communicate with him and make sure this sparks up your relationship with him and that he should understand that this is not just about you but both of you.
Media Specialist/Pop Culturalist/Eugenian/Native Louisianian/Professional Weirdo