Jah, you've already made it clear this isn't something you're genuinely interested in.
I know he wants to do it but I think this is something that all parties genuinely want to participate in because of the amount of trust required.
You know you're not 100% into it, you acknowledge dude was promiscuous in his past, and you're worrying about what kind of person to bring and don't want him to decide. You disclosing these details lead me to believe this might not be the best thing for you to do not only because you aren't completely interested in it, but because you have personal hangups about his past and you feel the need to choose the woman- not him.
Multiple partner sex isn't my cup of tea so I don't have experience with it. But that kind of activity definitely requires a high amount of trust and stability in the relationship to successfully do and maintain the bond because you're temporarily bringing in another party into the bedroom.
So hun, I ask you, do you really think this threesome is worth it? How possible are things changing because of this threesome? Will the trust still be maintained? Are you sure you won't have any jealousy issues?
You take the time and think this all out, okay? Because this ain't a simple walk in the park.