Sorry have only just seen this and send lots of good wishes to Riley
I suspect being at the vets is not helping him as was seperating him from Lucas.
I know Kocka was suddenly taken ill and then spent a week at the vets in isolation cos they were scared she had FIP or somethin g terrible...........she didnt and I visitred her every evening and in the end I said I was taking her home and they said it may have been a blood infection but never found out what was wrong with her.
I agree about the ad diet and it would always be the first thing I would try with a very sick cat but Kocka would only eat it off my finger but it was good to add bits of mashed tablet in.
I do hope he is improving and am afraid when Franta was in this situation about 18 months ago, I could hear him howling when I walked into the vets and the vet said he had trashed his cage............he just wanted company and to come home. Gradually he started eating again and xrays showed he had only one good kidney, the other was shrivelled and useless but Bristol said it was best to leave things as they were.............it may have been like that for many years.