Thank you Yvonne,
First, let me say that I am very glad that the paws are warm and that he can move them if he is stimulated to. This removes worries about spinal issues or blood clots cutting off circulation here. Furthermore, if his bloods were normal before the procedure, hopefully we can rule out systemic organ troubles here for Sockie.
With those ruled out, this suggests that soreness is likely to blame for his signs (especially since painful kitties often do just as yours had where he is too weak to use the legs and avoids anything --even the litter box-- that may cause them discomfort). Now his signs do seem quite severe for what he has done (which is why I wanted to rule out those other potential issues) but we can see some soreness post x-rays due to having to position the cats in positions (ie on their back or with their legs extended, etc) that they may not be comfortable to assume themselves. This can lead to irritation and exacerbation of their arthritic discomfort, even if it was relatively mild changes. And this may be worse for him if he isn't having any pain relief now due to his lack of appetite (which could have been from his feeling off from having an anesthesia or may be a bit of vicious circle since he may be too sore to bother wanting to eat as well).
Now in Sockie's situation, I would say that we do need to get him eating a bit so that we can use the Onsior. The reason why we don't like to use these anti-inflammtories on empty stomachs is because they can cause stomach irritation due to the drug's reducing the stomach's mucus production and making it more vulnerable to stomach acid. This leads to appetite decline, which increases the risk of irritation (since the stomach acid only has the stomach to "chew on" and this can lead to a risk of stomach erosions or ulcers.) Therefore, the first step is to try to get him eating and protect his stomach from any GI upset present.
Therefore, to start, you can consider giving him an antacid. This will help settle his stomach and protect it (by reducing stomach acid) if you choose to restart his pain relief. There are a number of antacids that are available over the counter and pet friendly. I would advise only treating with one, but the two I tend to recommend are Pepcid (More Info/Dose) or Zantac (More Info/Dose). This medication of course shouldn’t be given without consulting your vet if he does have any pre-existing conditions or is on any other medications. Ideally, it should be given about 30 minutes before food to ease his upset stomach.
Once we have settled his stomach, then I'd advise trying to tempt him to eat (as I am seeyou have been). Favourite foods are allowed or you can tempt him with a light/easily digestible diet. Examples of this would be boiled chicken, boiled white fish, scrambled eggs (made with water and not milk), meat baby food (do avoid the ones with garlic powder in the ingredients) or there are also veterinary prescription diets that can be used here (ie Hill’s I/D or Royal Canin’s sensitivity.)
Further to this, if tempting doesn’t work, then we do have to consider initiating syringe feeds to get food into him. In that case, you may want to try try Hill's A/D (LINK) from your local vet. This is a critical care diet that is comes as a soft, palatable pate. It is calorically dense, so a little goes a long way nutrition-wise and this could just help get some more calories into her even if we can’t get a huge volume of food in. As well, for syringing food, you can use the animal version of Ensure (balanced for animals dietary requirements) called Clinicare Canine/Feline Liquid Diet (LINK)). It is actually by the same people who make Ensure, but is formulated to meet out pet's dietary needs. Your vet should be able to order it for you but it is available without a prescription (some pet stores and even Amazon stock it as well). They also make one specifically for older cats with kidney troubles, and this could be an alternative for a cat his age.
Once we have some food in, you can then consider restarting the Onsior. If he hasn't had much response to it, then you will want to speak to his vet tomorrow about alternative pain relief for him but for the moment you can try using this again to ease at least some of his pain until he is seen. You can also start the ArthriAid now as well. The hope is that if we can at least address as much of his soreness as safely as possible until you can have you vet check him over and adjust his pain relief.
Finally, since you noted needing to syringe fluids into him, I do want to just quickly advise on how to monitor him for dehydration. To check this and make sure he is not becoming dehydrated there are a few things we can test at home. One is whether the eyes appear sunken, if the gums are tacky instead of wet/moist, and whether he has a "skin tent" when you lift the skin. To see how to check these parameters for dehydration, you can find a wee video on this HERE. ( They use a big dog but it makes it easier to see and the principles are exactly the same) If you are seeing any signs of dehydration already, then you do want to have him seen by the vet before this gets any further out of control.
And just to note, since you are syringing fluids, his daily allowance you would need to syringe would be 48ml per kilogram of his body weight daily. This, of course, needs to be divided over multiple offering over the day.
Overall, just as you suspect, all his signs are suggestive of pain post-procedure. His signs are telling us that despite it being a fairly non-invasive procedure that he does have some soreness, possibly from the stretching and positioning during the x-ray. At this time, you will be limited in what you can do at home to address this but to start we want to settle his stomach to get him eating and drinking better for you and then get that pain relief restarted to at least offset some of his discomfort. Hopefully, as we do that, we can get Sockie as comfortable as we can until his vet can reassess him first thing tomorrow and hopefully provide some pain relief that works better for your lad.
All the best,