So glad to hear this hasn't been/shouldn't be an ongoing problem. I re-read your original post and honestly, you don’t say that this only happened yesterday. Animal welfare is something that makes a lot of people very upset very quickly, you sound just the same or you wouldn’t have posted.
On the food side, I've often had cats with food issues, probably because I've always had rescue cats, fostering and my own, and the best I've found for them is Royal Canin, dry or wet. Wet is very expensive, yes, and I'd only feed it to help a cat get through illness if needed, but I can't remember any cat of mine rejecting RC dry, even those with very few teeth - I break it up a little with a rolling pin for those if needed - and there have been a lot of cats. I’m 70 now and have always had around five cats at any one time although now I’m older and disabled, it’s down to two.
If your cat is older than about seven years, and there’s no particular health issue diagnosed, try him on RC dry for elderly cats, introducing it gradually and always have fresh water down for him. It's low in phosphates which helps an ageing cat's kidneys. I start mine on low-phospate food even if they're not showing signs of kidney disease, which can include vomiting, at age seven, or what I think is age seven, bearing in mind he/she is a rescue cat. One of mine is currently on RC for liver problems as he has a congenital liver disorder which killed one of my cats at age 12 many years ago; he’s 10 now and thriving. If there’s a particular health problem I’d absolutely invest in Royal Canin’s dry formulation for that particular problem, liver, kidneys, anything else, I’ve had too much experience in feeding that kind of food and having my cats often live past 20 to do anything else now. Hills’ vet diets seem not to suit a lot of cats, maybe it’s because the biscuits are too big for many cats.
Mine get a chicken meal whenever I roast one, and fish no more than once a week if that, for a change. Fish is so expensive now, I buy frozen whiting or generic ‘white fish’ from the supermarket if they have it, otherwise coley. I take care to pick any bones out with tweezers, though, fish just gets hoovered in a flash. I’m a well-trained slave. 
There are lots of choices on the market in veterinary and non-vet specialist foods, though, and vets charge a hefty mark-up. I usually go to Animed Direct or Viovet but there are other good suppliers, always looking around for special offers first. Disclaimer: I have no connection with any company in this field, just a customer.
Apologies if you know all this and I’m telling you to suck eggs. I hope your cat’s problems can be solved quickly. A sick cat not being able to tell me what’s wrong just breaks my heart.