Going back to the OP's request for some advice, I'd like to offer up my experience as both a cautionary tale as well as some how-to guidance for how I have been creating a poly/mono relationship with my mono partner of 5 1/2 years.
When I first broached the topic of non-monogamy with her, she threw a coffee cup at the wall. Granted, she has a temper, but this was a real indication of her initial feelings about it. We had been together about 3 years at that point and married for one year. I didn't bring it up again for another year. I was very unhappy during that time, swallowing most of my feelings down and feeling like I couldn't talk to her about what I was going through inside myself, coming to the full realization and acceptance of the fact that I am poly.
When I did bring it up again, it was because I eventually cheated on her. (With someone who smelled really damn good to me!) This was horrible, and obviously the worst possible way to re-introduce the subject. But it did force me to get real with myself and with her, and look at my situation directly. We had some really difficult conversations after that, where I basically said, "Look, I did this terrible thing and I never, ever want it to happen again. I can no longer be authentic to who I am and promise you that I will never be with anyone else. But I can promise to tell you the truth, and to honor your feelings and work through this together."
Long story short, here we are about a year after that with a bumpy but progressing poly/mono relationship and very explicit agreements. I ended up having an ongoing playdate relationship with the person I cheated on her with for about 6 months, and have recently started seeing someone new.
The advice I would have for you if you are going to embark on these conversations and really want it to work, is to be very patient and understanding, and at the same time be as honest and authentic as you can be. This builds trust, and also gives your partner the room to have his feelings and know that you can sit with those. Personally, I'm still trying to find the balance between sitting with someone else's difficult emotions and feeling compromised or blamed in some way, so I can't give you advice on that. But I can say that with my mono partner, being willing to spend a LOT of time listening to her and really helping her to feel heard has made all the difference in making progress and (re)building trust. Also identifying her particular fears around poly stuff really helped. Like for her, it wasn't about the sex but it was important to establish (and repeat again and again) that I was not looking to replace her, that she is the only one I want a primary relationship with, the house, the marriage, the dog, and maybe someday kids with, etc.
I hope that helps. Best of luck to you!!