He is 15. He has a tumour behind one eye, but his kidneys are also failing, so no operation/treatment for the tumour. Basically we're just monitoring him and ensuring he has a good quality of life, before we accept the inevitable and have to go down the 'PTS' route 
Since around March he has been on a prescription-only diet from the vet. First brand of food (can't remember which it was) he really didn't like, despite us introducing it very gradually. Currently feeding him Royal Canin; chicken and tuna varieties.
He doesn't really like the food though. He eats it, but he doesn't eat very much at any one time. Rather than feeding him just twice a day, as we used to, now we feed him a little bit several times a day, to try and encourage him to eat more. But he leaves a lot, and often just licks the gravy off the chunks. (He always has plenty of fresh water alongside his food)
I'm a bit worried about how little he is eating, and he is definitely thinner.
I know that with failing kidneys that he needs a low protein diet. The Royal Canin is 6.5% protein. I had a look at the various different foods in the supermarket today, and a lot of them are around 11-12%, so obviously not great. But I bought a couple of sachets of Sheba Fine Flakes today, which are 8%. I fed him one of those this evening and he wolfed almost the whole 85g pouch down.
So - am I doing the wrong thing by feeding him this? I can't help feel that if we're going for quality of life, that he ought to be enjoying his food. But obviously I don't want to be feeding him something that might be making him worse/feel worse 