Your feed store should carry some antibiotic like Penicillin and [Tetracycline] maybe under a different name so read labels,If the infection is in the joint it most likely will not be resolved by it self. But maybe @casportpony can give some advise i like to refer to her when it is about antibiotics and what is best to use, yes he may have hurt his knee when he was limping just hard to say.
This may help
https://www.backyardchickens.com/t/...gals-coccidiostats-and-wormers-preliminary/10