It doesn't have to be complicated. My point in listing all that is do NOT be afraid to say "NO, I do not want to Open again to include another person. This is plenty overwhelming as it is with 3 people in here!"
If more people would just own it and speak it straight up from the beginning? Then they wouldn't suffer later because they were too afraid to speak up and just put the brakes on it. They went against their own willingness and their own best healths for fear of "seeming selfish."
Well, it is what it is. THIS is what you can handle AT THIS TIME. No more, no less. Here is the limit right now.
YOU are responsible for your own well being. So just say no. "I'm happiest as the V in a closed polyship at this time. I do not want to be in a 4 person polyship. Too stressy to me. I'm not willing at this time. I am not ready at this time. "
There. Done. He can then make his next choice based on that information. You sent it off to his desk. Until the memo comes back on to your desk, it isn't your problem any more until it is your problem again.
I think you guys could sort out what you want for open relationship models.
Talk it out.
It seems part of the reason you feel overwhelmed could be that you guys did not talk enough about the HOW to be in polyship together before jumping in. Could sort that out too. http://www.practicalpolyamory.com/im..._Polyamory.pdf
Hope you feel better soon.