Hello, and welcome!
I'm sorry your cat isn't feeling well and it seems that he ate part of a sock. This can cause an intestinal obstruction, and he won't be able to pass any stool, if there are more pieces of the sock. Sometimes he can get rid of a 'foreign body' (the sock) by himself, and sometimes, he needs the vet's help. You can give him some hairball remedy gel like Laxatone or Petromalt to help any remaining pieces of the sock, hopefully move through his system. There is no way to know at this time, whether it will pass on it's own, so it would be best to bring him back to the vet, asap. They might do some imaging, like an x-ray to see what is in his intestinal system. He may have swallowed some other non-food item, too.
Also, if he's not eating, it can be due to his fever and the antibiotic, but if a cat doesn't eat, even for a short time, he can develop a serious liver condition called hepatic lipidosis and we would want to avoid that at all costs.
Continue to monitor him and if he seems to worsen, bring him to an ER vet tonight. If he stays the same or seems a little improved, you can wait until tomorrow morning and bring him to the vet, first thing, when they open.
I hope all will be well with your furry boy!
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