I have a cat who went through the exact same, the cat was skin & bone, I gave worm tablets I fed mince steak etc. to no avail, until she showed me her hidden kittens which she was feeding.
Now I'm (seriously!) not saying that this is the case (judging by age etc.) but it would make your vet look, well, quare silly
.
Hope he improves tho'. Cats are so sociable with each other that there is no knowing what other cat is around spreading flu or the likes. Ask the SPCA if they know a vet who is knowing about cats (some country vets pooh pooh at family pets & only want a horse or a cow).