Rather than buy bottled water or the britvac filter, spend out on a pet water fountain. Most cats prefer to drink running water and these fountains have filters so will get rid of all the nasty stuff in the water. Water will definitely help what is wrong with him. If you were told to go with dry food why don't you try a mix of wet and dry food? Get a wet food with a high water content that is low on additives (like gelling agents or sugar.) There is a cat food brand called Encore (I will need to double check the name but this is what I remember) that is 75% chicken and 25% water when compared to other brands that is 68% chicken product with a mix of rice, water, additives, gelling agents and more blah so is generally healthier. Mixing it with dry food should make it better all round for your cat. Might also be worth looking into a pet insurance as they may stillinsure your cat because of it's age. It will work out cheaper than a hige vet bill because you are spreading the cost but you would need to check as to whether they will be able to help with a pre-existing medical condition.

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Run free my beautiful rainbow babies. I love you.