I'm sorry your friend isn't well. . .can you tell me, how old is he?
Is he on any medications or fluids?
When was the lat time he had bloodwork done?
How is his appetite?
Yes, I have. It is most often associated with anemia, which is part of the progression of renal failure. So, I am concerned that your friend may be anemic. At home, you can check the color of his gums. They should be pink, or pale pink. However, this is a very rough indicator - much more useful would be to take a small blood sample to check his Packed cell Volume (PCV). This is the percentage of red blood cells per total blood volume. Your vet can do this easily at the office.
The kidneys normally make the hormone erythropoetin that tells the bone marrow to make red blood cells. When the kidneys aren't functioning well, this hormone is deficient, and there are not enough red blood cells being made.
I suggest that you check all of his kidney values at the vet's. If his results have gotten worse, you can discuss thing to do at home to help him. Did your vet ever talk about giving him fluids under the skin to help his kidneys?
There are things which can be done to help him. If he is an anemic, there is even the option of injections to replace the hormone. Some cats are good candidates for this treatment- you can discuss this with your vet.
I hope this has been helpful. Please feel free to ask any other questions you have about your buddy. I have a lot of experience both professionally and as a cat owner with dealing with kidney failure.