Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
Poor wee Sooty.
Now Sooty's decline in vocalizing is not a specific sign in itself, but fits with the non-specific lethargy that suggests he is feeling poorly at the moment.
Furthermore, I wouldn't expect falling off the bed to have precipitated his current signs. That said, I am a wee bit concerned about what lead to him falling off the bed. The reason is because we can see renal disease cause metabolite (ie urea) build-up in the blood that can lead to mental confusion, depression and even seizures (which possibly could have been what caused his fall).
With all that in mind, we need to consider his other signs, the appetite decline and continued depression. Both again do raise red flags of whether he is being affected by the build up of metabolites/toxins that his kidney's should be filtering. When we see these levels start to rise we can see the change to thirst and urination that you noted, the mental/brain based signs I mentioned, as well as gastrointestinal signs. In regards to the GI signs, we can see nausea, appetite decline and anorexia due to secondary uremic gastritis. Furthermore, we can see kidney cats become too sore to eat due to uremic ulceration erosions of the mouth and throat. So, these would all be concerns for his wanting to eat but then not being able to do so properly.
In this situation, with him quite depressed, it would be ideal to check his kidney levels (urea, creatnine, phosphorus) via blood sample to appreciate how severe they are. If they are very high, then IV fluids may be indicated to flush out the toxins and get him feeling better.Alternatively, if the changes on bloods are mild or as a longer term management options, I would note that it is possible to give cats fluids under the skin (subcutaneously). If you are interested in this, you can find a good outline HERE.
As well, to try and offset the nausea/gastritis his kidney disease could be precipitating, we do have a few options. To start, we will often treat them with an antacid. There are a number of antacids that can be used to help kitties in this situation (ie Cimedtidine, Pepcid (LINK) or Zantac (LINK)) and can just make eating and drinking easier for them.If they are really struggling, then his vet can treat him with injectable anti-vomiting medication. And if uremic ulcers are visualized in the mouth/throat, then your vet can use kidney safe pain relief (ie Bupenorphine) to help address
Otherwise, you didn't note whether he was on any medications but I do just want to make a quick note that we often will support kidney disease with medical management. To do so, we support kidney filtration using ACE inhibitors (ie Fortekor) or treatment like the newly marketed Semintra and phosphate binders (ie Renalzin) to help remove phosphate build up commonly seen with this disease. If he is anemic, there are drugs the vet can administer (ie Laruabolin, Epogen, etc) to stimulate his marrow to produce red blood cells (which normally would have been hormonally stimulated to work by the kidneys).
Overall, I am very suspicious that Sooty's signs are a hint that his kidney disease is taking a toll on him and that the metabolites are building up in his blood to a level that is making him feel rough. Therefore, it would be ideal to consider an antacid to settle his stomach just now but also consider having his renal levels checked at this stage. Depending on your findings on his bloods, IV fluids to flush out the toxins may be necessary or at least subcutaneous fluids at home +/- medical management to aid his kidneys, get these toxins out, and get him feeling more like himself .
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,