Hi sorry basil is poorly, we had our cat ill a few months ago, similar to you boys symptoms. He was tested for a lot of things, he wouldn't eat at the vets, but as soon as he got home he ate lots. The vet said he was too stressed in there so it was best for us to bring him home.
He is a very food driven cat.
He had a couple of dose of appetite increase medication. He soon gained weight, but it took him some time to get back to being his usual self.