Most of us here are going to need to resort to other means of healing than antibiotics, it appears. After all, it's really the immune system that is responsible for most of the healing. The antibiotic is only to target the bacterium and take shots at it, reducing the numbers so the immune system can kick the rest out.
We'll need to read up on homeopathic/herbal substances such as St John's Wort, which you might consider giving to your patient as it can help with lameness. Others such as Echinacea and turmeric can help with infection. Research the internet for more. But before you give anything to your chickens, search to see if it is a toxin to poultry or you could end up with a poisoned chicken.