I don't have a lot of advice, but I wanted to drop a few sentences your way. If you DO decide to try this, yes, meeting a new person (whether friends or lovers) doesn't mean the old people get left. Even if you think they are better looking, even if this new friend is funnier, or more into mountain climbing. That is one good thing about poly. The fact that it sounds like it's been being talked about for awhile is a good thing, knowing that he isn't rushing into something helps.
If you do venture forth, you might want to identify just what your insecurities center round, and help give him ways to address them. Otherwise he might be doing or saying what he thinks will make you feel better like constantly telling you he loves you and that you're special, when it doesn't help because what you really want or need to hear is that he's glad to come home to you at the end of the day because no matter how crazy his day is, he feels at peace when he comes home to your smile.
Also, unless he has said he wants to have kids no matter what, there is no reason to think that just because he dates other people that he will have kids with them. It does not mean that other partners will live with you some day in a big family so you'll have to help raise children he has with his other partner. What it means is you can negotiate anything you want. If staying with him if he wants kids with somebody else is a deal breaker because it would break your heart, it's OK to say that. If you decide you would try it if biological kids are really important to him...well that's at least a couple years in the future isn't it? You've got to date people, like them, love them, decide if having a family makes sense, make sure nobody is rushing into parenthood too fast. You get to figure out if you want to date or not, etc.
Again, my couple of sentences ran into a big old essay, but mainly I wanted you to keep in mind that poly have thousands of configurations, and you do get to decide what would or wouldn't work for you.
Happiness will never come to those who fail to appreciate what they already have.