Cat has been sick for month

Hi. I'm new to this site just curious if anyone has had a cat with the same symptoms as mine. He was originally very lethargic, had a little runny eye and not eating so I brought him in to see the vet. She gave me antibiotics which did nothing for him. He progressively got very congested, still has a fever, his eye became very red, and was not eating much. He has taking 2 different kinds of antibiotics, antiviral, and eye drops. His eye has cleared up but still not 100% but vet said it's ok for now just keep an eye on it. He is eating better but still not as much as he use to and he is drinking. He uses the litter box and goes the bathroom. His poop is very small but at least he's going. He has had blood work done more than once and X-rays. He's been tested for multiple things but still no answers. They seem to be stumped. The next thing is having an ultra sound done on Wednesday because he still has a slight fever and congestion. It's all just extremely frustrating and very costly. Anyone have any insite?

how old is your cat?  did you change food? could your kitty have gotten into something to make them ill?

has he been tested for all the bad stuff? coronavirus? herpes? etc

He's 6 years old. I've been trying to give him mostly wet food since he wasn't eating the dry. I've seen him nibbling in the dry here and there. I don't think so. He's an indoor cat. I haven't noticed anything that he could have gotten into. Yeah they did all those. Vet said fip still isn't out of the question but after talking to a couple internists they said doesn't seem to fit quite right. I'm really hoping that's not what it is.