Talk to your vet about the antibiotics. Some antibiotics can depress the appetite or if this happened right after he finished the antibiotics it may be he still has some infection bothering him.
Try tempting him with chicken baby food or fresh cooked chicken or any other meat he might like to get him started eating. You can even warm the food up to make it more aromatic in case his nose is stuffed up.
Cats can get a liver problem if they fast too long so talking to your vet or having the cat checked over for any new problems is worthwhile doing soon.
Might be something totally unrelated to the antibiotics such as a bad tooth or an upper respiratory disease bothering him.
With my old cat when he went off eating for a bit we convinced him to eat by using things like some of the meat from a shaved steak sub, smoked turkey, and any other meat we were eating that he showed an interest in. Gradually he got back to eating again. Turned out he had swallowed plastic fronds from a silk flower arrangement.
Hope this helps you!
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