SO Bunny actually got namedropped to Yuya back in Venera as someone who was gathering infected to keep them safe, so he already has a positive impression of him. It wouldn't be hard to convince him Bunny is actually here to help and given Yuya's lack of combat ability having support from a Kenoma that they can work together to rescue people would be pretty ideal, if you're up for working out a way they can be convinced to collaborate?

Or tell him he doesn't have to fight for the regent he can join them Whenever He Wants that might give him something to mull over for the future.