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It sounds very much like he has an infection probably resulting from the wounds he received - or more seriously he has been contaminated with an illness something from the other cat He really needs to be taken to the vet for an examination and prescription of antibiotics or medication- there is not a home remedy you can use for this situation.
You do want to get him in as soon as you can because this will not get better on it's own - it already sounds like the infection or illness is 'deep' in him and waiting much longer will only make him get worse.
I wish you all the best with your cat!