There is a very good chance that the signs you are seeing are related to something called a saddle thrombus. This is a blood clot that originated in the heart and was ejected in the bloodstream and ended up devitilizing the rear legs. You may notice the back feet feel colder that the fronts. This is a painful condition and can occur in older cats. Your vet option is to see an ER vet despite the bad timing. Your vet may use heparin to break up the clot. In some cases, surgery can be performed to remove the clot. Either way, the prognosis with this is rather poor and many cats will have repeat thrombi after recovering from the first. If an ER visit is not possible, you can give a 1/4 of an adult aspirin (325 mg) or a full baby aspirin (81 mg). This is a bit of a desperate move and only do this if you notice the cold feet on the rear.The anticoagulant side effect of aspirin could help break up the clot. I would not count on it working, but that is really the only "home remedy" that is possible at this stage other than trying to keep hims hind end warm.
If this is not a saddle thrombus, it could be intervertebral disc disease, a tumor, toxic issues etc. However, the acute nature of this combined with the signs of pain do support a saddle thrombus. Unfortunately the prognosis with a saddle thrombus is typically poor to grave. I hope this gives you some direction in what must be tough to watch.