Maybe let him sit with the idea for a month or two, them bring it up again. Sometimes people need time to mull over a new idea before it begins to seem less scary. Next time you bring it up, let him know that you're poly. Give him something new to sit with. If he comes around in his own time great, if he's still against it personally, let it be.
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.