"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 (23+ yrs)
Dude: hetero poly male, live-in boyfriend (4+ yrs) and MrS's best friend
Lotus: poly bi married female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe