Welcome to the forum both of you! LOL
You are most definitely not the first person who has had to ask themselves whether they are poly or just looking for others because their relationship is bad.
The concept of "full on poly" is not one that will achieve a lot of resonance on here - poly is what you each want it to be - there is no certification program, no universal ideal for a poly configuration. If you don't want someone to be a part of your family, then that's absolutely your right, and everyone involved needs to respect that. If he wants someone to be part of your family, then everyone needs to respect that too.
We each have our needs, wants and likes for our relationship life. The challenge is to try to get to a point where everyone gets at least their needs met. If they can get a goodly number of "wants" met, then that's good too.
If you and he have very different bottom-lines, then you are not going to be able to make it work.
Another poly adage that's out there is that you shouldn't fall into the "relationship broken, add more people" trap - it rarely, if ever works, and usually just adds drama and more tension to an already bad situation. You have to get the current relationships stable before adding folks. That's the fairest to you, but also to the people coming in. Otherwise the other people can become punching bags or negotiating chips - and nobody really wants to be that.
Your question "Why become poly" - ever since I was a teenager I didn't see why I had to love only one person - why everyone else that I cared about had to disappear (at least form me heart) as soon as I got into a relationship. Conversely, I stayed friends with all of my exes, and couldn't understand why, because our relationship wasn't working, I had to suddenly "hate" this person when they hadn't done anything wrong. When I spoke with other folks about this idea, it seemed utterly foreign to them, while it seemed completely natural to me. I think that was my first clue that I had stuff in me that was "not normal". The rest is a long and painful story that I have written about quite a few times. For me, while sex is a part of a relationship, it's not about needing more sex, or different sex. For me it's about love and romance with multiple people, not just a way to balance differing sex drives.
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb