"Relationship broken, add more people." Is not generally a formula for healthy relationships.
I would strongly consider stepping back and have him sort out his issues with his girlfriend before negotiating what an open relationship with you will look like.
It's great that you have learned something about yourself from this experience - i.e. that you are not as opposed to "sharing" as you thought. But transitioning to poly/open takes a lot of effort/personal growth for many people. Everyone involved should, ideally, be starting off from a good foundation.
Clearly, at this point, he has other stuff going on - staying vs. finishing relationship with current girlfriend - so, even if mono/poly could work for the two of you - this doesn't seem like a good starting place to come from. He might be tempted to agree to a certain type of open relationship with you just because it looks like a way to salvage a non-ideal situation.
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (together 21+ yrs)
Dude: hetero poly male, live-in boyfriend (together 3+ yrs) and MrS's best friend
Lotus: poly bi female, "it's complicated" relationships with Dude/JaneQ/MrS
TT: poly bi male, married to Lotus, FB with JaneQ
VV and MsJ: bi-women with male primaries, LTR LDR FWBs to JaneQ
My poly blogs on this site:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe