It is a request. He hasn't made an ultimatum, but he has said if we can't forge an agreement, he isn't sure what will happen. He wants to be with me, but he wants to pursue his sexual interests.
Request is good. It's not a demand.
He doesn't seem to have any idea what type of open relationship model(s)
he resonates with.
At this juncture you are well within your rights to say "Need more data. This offer on the table you present me is lacking information."
Again, could thank him for talking to you FIRST.
Could read things together to gather data together and determine what kind of specific offer of open model relationship he's presenting you.
When you get to the place of giving your Final Word -- could still say "No, thank you. I do not wish to participate. It is not for me. But thank you for treating me well and with respect."
You do NOT have to do things you do NOT find appealing or wanting to do.
He wants to be with me, but he wants to pursue his sexual interests.
This could use more data also. What is "pursue his sexual interests?" Where's the agreement line there? Participating in online kink communities? Going to a dungeon to voyeur and nothing else? Or does he mean have kink play partners he shares sex with?
If the details of his offer cannot be clarified?
Then the conversation may have to change from "Can we talk about Open and what that could be?" to "We cannot Open and be together. We are not compatible on this because we want different things."
Then he has to decide if he wants to give up the want to be with you, or give up the want to be Open.
Talk honestly -- and sort it out.
Hang in there!