I had been dating Gia in a secondary-type relationship for close to two years when I decided to re-start a relationship with my ex, Davis, with the intention of aiming for a primary-type relationship. It isn't quite working out that way, but that has to do with issues been he and I, not with my relationship with Gia.
The idea that everyone who wants a primary already has one is preposterous. Even if it were somehow true, what about the many people going through break-ups or divorces at any given time, who don't choose to give up on the idea of a life partner afterwards?
And hey, if he wants a poly-friendly partner, this way he will *know* they're ok with it, versus just thinking they're ok with it without knowing how they'd react to the reality.
Your bf's position may not be the norm, but I don't see any reason at all that it can't work out.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.