We have one Persian cat (2 years old) with a history of kidney disease that led to Feline Lower Urinary Tract Disease.
His food is similar to this kind (which is prescribed by our veterinarian):
So I know this solid food is fine for him.
I also give him almost half a glass of milk, so is the milk also fine for him? If not, can I give him some other liquid like milk?
For a cat with kidney or bladder problems, you need to feed wet food so your cat gets the water it needs.
Feed the type and amount of food your vet tells you to; to save you some money, it might be best if you buy canned food -- it has a lower cost than the portion packages you can get.
If your cat loses or gains weight, you need to ask the vet what amount of food you need to give your cat.
Be sure to only give the food the vet has told you to use -- no treats, no milk, only cat food and water.
Cow milk contains lactose. No adult animal has the need or use of milk, and in some cases milk can do harm to the body. If one gives milk to cats it can lead to diarrhea; diarrhea is particularly dangerous to cats with kidney problems due to the loss of water from the body.
In addition to the loss of water, milk contains calcium, phosphorus, and fat. These are things that put extra stress on the kidneys and might lead to bladder stones, kidney stones and gallstones.
The excuse your cat likes milk is a really bad one for giving milk to a cat.
I myself like a lot of things that can cause harm or even might be lethal, so common sense makes me avoid them.
Edit: the OP have changed the question a bit after I posted this answer, but I leave my answer as it is.
