If you really want to try to make this work, take some time to see if it might be possible. Give him the opportunity to talk this through with you, learn about poly (www.morethantwo.com
is a great 101 resource), and feel out his initial knee-jerk negative reaction. Maybe that strong negative reaction will always be there, and this is doomed. Or maybe it can soften with time.
Davis (see my signature line) was dead-set against poly when he first learned of it. He thought it was a bad idea and could only lead to heartbreak, and would never be for him. Well, it's still not for him -- he's mono to the core. But he's come to accept that it IS for me. We tried a serious, aiming-for-primary type relationship while I was also seriously involved with Gia. That attempt didn't work out, but for reasons that had nothing to do with poly. Now he and I are friends and secondary partners who love each other, and he's fully accepted my polyamorous heart. He actually told me a few months ago that he'd come to see poly in a much more accepting light, and that he wouldn't mind meeting my other bf some day (this despite the fact that we'd had some MAJOR friction over my decision to go barrier-less with that other bf, which had made me assume Davis might resent him).
It'll likely be a long road, and you need to prepare yourself for the fact that his feelings might be inflexible. But mono/poly relationships aren't impossible.