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The limp and the oozing are likely related to the an abscess. These are common problems with feral cats. Because this cat will be tough for your to handle, you may have to try to use a live trap (animal control or your vet clinic may have one to borrow). There is no over the counter antibiotic that I can recommend at this stage. It is standard of practice to have the animal seen before an drug is prescribed. However, some vets will let this slide in situations like this so it will not hurt to ask. If you are able to catch him, your vet will also be able to clean up this wound as well as use oral or injectable antibiotics. I hope this helps and gives you direction.