I'm new to the scene, so take my advice on this with a grain of salt. Hopefully seasoned polys will be able to help more.
It sounds like you're asking how to handle jealousy, so that is what I will speak to.
This advice was given to me by a member here, on the same topic.
When you feel a reaction bubbling, pause. Examine it, re-examine it, dig deeper & find the root. For myself, and others as I've read, the jealousy/envy comes from an unmet need/want. Find that. Then pause again. Collect your thoughts, organize them. Calm yourself. Explain your thoughts to your partner & get his reaction. Pause AGAIN. Examine his words, and his meaning. Even though men are very direct creatures, sometimes their words are not exactly their intention. I'm guilty of that one myself, I know what I mean to say but it's hard to put into the correct words. If you're unclear, ask, CALMLY, what exactly he meant. Gather your reaction to that, organize it, etc. Then discuss, openly & calmly, a reasonable compromise. Find the baby steps that work for you.
Even as the instigator of the idea, it's a difficult transition. Take it easy on yourself & him. One of the best things you can do for yourself, your relationship with him & every relationship you have now & in the future (family, friends, romantic, whatever) is to accept, conquer or at the very least recognize the insecurities you have so that you can learn to appropriately deal with them & understand your reactions in a more logical, less knee-jerk kind of way.