What's the best food to feed my 8 year old cat who has upper respiratory issues and a history of UTIs?

Depends on what his allergies are - many many cats are sensitive to corn, and corn's a leading cause of upper respiratory issues (I have one of those cats). Many to most cats with urinary issues can be very successfully maintained on an all canned diet. You can surely try that. Ditch the dry - all dry even the prescription one. The canned is still crummy, but does contain less filler than the dry. Dry is the leading cause of UTI's anyway. You can try a quality, no filler canned commercial diet and see how he does. If the UTI comes back, then use only the canned C/D (Royal Canin's SO diet is higher quality BTW). And see if his respiratory issues don't improve! Good luck with him.

High protein, low carb, no corn in it, few grains. No by-products if at all possible. You need good food to keep the cats immune system up. The UTI's can be helped by encouraging water intake. Set a mug of water around the house in odd places, the cat will drink more because they think they're getting away with something.

Science Diet, Royal Canin are good. Really you can't go wrong if you stay in the premium food arena. Even some Purina types are fine though. Just don't go super cheap, generic. Make sure meat is the first ingredient and then grains. The more meat the better, they are carnivores. If you can't afford the premium foods then I'd recommend Purina brand, get an age appropriate food, probably senior food.

what allergies does he have, and how much water does he drink a day? Is his food a hill science one?

If if drink's the right amount of water then uti's should not be a problem really? cats drinks 5fo to 10 fluid ounces per day. Now cats that eat wet food daily will need like 1 ounce plus of additional water per day. Whereas a cat eating dry needs over 7 ounces plus per day to stay hydrated. If the cat is drinking more than enough he should not suffer uti's and should never be fed dry food of any type

catInfo.org (click on urinary tract diseases on the black side bar)

With his upper respiratory problems read vca animal hospitals .. Any signs of uri can mean the cat is a carrier or has got a bad uri( rescue cats can get calici virus etc.. If you know its not any of theses and know its from food allergies alone then change wet food every like 10 weeks to see if there is a improvement keeping tract of ingredients.. I don't think with it being a shelter cat it would be from food allergies? look in to natural cure websites for products to boost your cats immune system and products that are going to help your cat with his exact problems.

Hills diets are not good for any animal but when its a sick animal, its not what goes in to the food that is a problem.. most of the time its what is not in the food. Animals fed on theses diets to cure 1 problem normally end up with a totally different 1 due to not receiving the right nutritional requirements etc.

I had a cat with calici virus and it started with sneezing running eyes etc nothing to bad(vet said allergies for over a year?) then they gave him his yearly vaccines and he went down hill fast from sneezing blood to not being able to breath.. You need to read up on to boost the cats immune system overall good luck as I had both these problems with my rescue cat