A 6-inch bunny typically doesn't need any help eating. And no, they don't eat anything you can find at a pet store. After all, in the wild there isn't any Bunny Chow. Lots of grass is best. Are there any puncture wounds from the cat? If so, the REAL danger is infection. The bunny needs to be on antibiotics immediately.
Please contact a licensed wildlife rehabilitator. Check here for your state to find a rehabber.
http://www.tc.umn.edu/~devo0028/contact.htmIf you cannot find a rehabber, start calling vets. And while you are looking for the best help for him, KEEP HIM QUIET, KEEP HIM DARK, KEEP HIM AWAY FROM ALL PEOPLE, NOISE AND PETS. Marybelle is absolutely right; the stress of captivity may cause him to die within 48 hours. I have seen them keel over and die just from a loud human voice.
Nancy, licensed wildlife rehabilitator who specializes in rabbits.