Can you help with feral cat?

Hello. 2 1/2 years ago I wanted to get my dad a cat for Christmas. Mom went to the animal shelter and brought home "Bruce Willis". He was 2 years old and found in an abandoned barn where he appeared to have lived since birth. I didn't know anything (and still don't know much) about feral cats, but I thought we could love him enough to make him our new kitty. We give him all the love he allows us to, but he continues to attack our feet quite aggressively at times. He will allow you to pet him, but only as long as he wants then he bites. Hard. I was recently diagnosed with diabetes and I can no longer tolerate the bites/scratches to my feet. We have tried since we got him to break him of this habit, with no success. I would like to try everything possible to help him so we can keep him, but I'm at my wits end. When he bites, he draws blood. Is there any hope for Brucie?? Thank-you in advance for any advice.