Maybe you could arrange for you and your boyfriend to hang out with your good friends and their girlfriend, and get them talking about their poly relationship as a way to bring the subject up? You could even tell them your aim beforehand so that they can help steer the conversation to whether you and the bf would ever like to try it. But I still think it's dangerous to go down this road if you will continue to have trouble bringing up certain things. Clear, fearless communication is essential.
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.