I would have recommended that website too.
The food is not only low proteine but also low phosphor and I am sure your vet can tell more.

Filou got the Hills k/d (wet) first - and hated it.
Now kidney kitties shouldn't lose weight so I was very worried. Now he gets the Royal Canine Remal in wet and in dry. The wet food comes in little pouches - which is a good thing as Filou will not finish a small can in one day. The dry food is so yummy even Filou's brother is crazy about it

You said that Cosmo lost a little weight- try to get it back on him (unless he was overweight). And please make sure the weight loss was not due to dehydration. As kidney kitties can't concentrate their urine as well as healthy cats, dehydration is a danger.

Filou gets weekly sub-q fluids.

You will learn from that website that kidney failure can't be healed- but with the diet plus (maybe later) the sub-q fluids the cat can still have some very good and normal years. Filou is now in treatment for 15 months- I hope I have him around for years to come.

All the best for you and Cosmo.