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He might not be eating due to acidity from the kidney issue.
You might ask your vet about using Pepcid for him to see if that works to help him eat better by controling stomach acidity.
You may find this site http://www.felinecrf.com/
in particular the page on meds in this instance
My feeling is the cat eating is better than not eating. So what we do here is buy a small variety of really premium canned cat foods to offer plus some fresh foods such as chicken sometimes a little hamburger seems to appeal.
This week our 19 1/2 year old who is in kidney failure says the Turducken from Merrick is the best but it varies day to day what he will eat.
Hope this helps you!
Secret with the cat food is mush it up and add some warm water before he gets it. The extra fluids are good for him anyway and this makes it all 'gravy'.
Fancy Feast is the only grocery store food we use and we don't buy the gravy type but the regular one and mush it up to make our own 'gravy'.
Buy one Blue Buffalo, one Weruva, one Merrick, one Innova Evo and see if he likes any of it mushed up with warm water. I try to stick with the less fishy foods but sometimes that is what works best - salmon works with him sometimes.
I usually do cooked meat as I worry about bacteria. We actually offer a little of whatever meat we are having to see if he is interested. (Yes we cater to the cat! )
And do read through the CRF site - full of helpful info for coping with this problem.
Thanks for the above accept and good luck with him! 15 is a good senior age for a cat to get to also!