"The only problem I see is that the Bf is still in a lose lose position"
Um, or, third option -- the work that he and the OP do together, their experimentation, the trust and honesty, etc, actually helps him to become a more fulfilled and secure and less jealous person, which serves him well as a human being while also negating the problem posed by his reaction to the other guy?
I'm not saying that scenario is a guarantee by any means, but there ARE more than two possible outcomes here! Seriously. Why assume it has to be lose-lose??
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.