" I am severely uncomfortable with this idea as I'm the one who would be giving up the exclusivity of this relationship".
Why just you? I know the whole thing wan't your idea, and I can see that a threesome is what he wants....
But, you don't actually have to be a part of it unless you want to. He could find two girls who are willing, and you?
You could find a guy that you are attracted to to go on dates with when your other guy is busy.
Consider it, at least.
Because your solution of picking the girl?
Really not advisable!
I come at this from the angle of the possible third, mostly because I'm not married and my relationships mostly fall into the usual "secondary" pattern. Any woman approaching me as a possible partner for her guy and possible third would have a horrible time getting me to even consider the idea. The situation just SCREAMS jealousy and boundary issues, and I am so very not into complicating my life in that fashion.
Let me say this again, anyone you approached to add as a third would see this as Drama Waiting To Happen, unless they were way inexperienced or not perceptive enough.
On the other hand, if you let him find a girl or two(please don't force it to be a package deal!) and find someone to spend time with of your own choice, this seems way more reasonable and less Inevitable Drama to most.