Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I am very sorry to hear about Matt's lethargy, weakness in his rear legs and pain and loss of appetite along with rapid breathing and drooling.
The most common cause of his symptoms would be a blood clot thrown from his heart blocking the blood flow to his rear legs (saddle thrombus). Are his back feet colder to the touch compared to his front feet? Do his back feet pads look purple or blue in color compared to his front feet pads?
These cats usually have heart disease that alters normal blood movement through the heart which allows clots to form in the heart. If he has a heart murmur, abnormal heart rhythm or a very fast heart rate I would be suspicious of heart disease and a blood clot causing his symptoms. His rapid breathing can be related to pain, but can also signify low oxygen due to poor blood flow secondary to heart disease. In most cases with these patients on blood work we will see high levels of CPK (creatinine phosphokinase) due to muscle tissue breakdown when the blood clot cuts off blood flow to muscles in the rear legs. Unfortunately prognosis for these kitties is very guarded due to severe underlying heart disease.
Weakness in the rear legs can be due to electrolyte abnormalities or low or high blood sugar, so blood testing should be done.
Trauma is another possible cause. Does he go outside unsupervised?
Some sort of myositis or neuritis, an autoimmune type reaction that can sometimes be associated with a vaccine, such as Rabies is possible. Was he given a vaccine recently? In those cases steroids would be helpful to quiet the over-active immune response as well as decreasing painful inflammation.
Finally we do need to worry about spinal cord tumors or intervertebral disc disease (the spongy cushions between vertebrae which can prolapse and put pressure on his spinal cord), which because they are soft tissue they would not show up on a radiograph. We may only be able to diagnose these with an MRI.
At this point I highly recommend that he see a veterinarian as soon as possible.
If he doesn't have any signs of heart disease, his paws are warm, his blood tests are normal, and you are not willing or able to put him through any further diagnostic testing, and understand that the improvement may be temporary, then a course of steroids may make him feel better and give him a better quality of life for a period of time.
You still need to rest him and confine him but steroids may help him either recover or at least feel better for a period of time.
Please let me know if you have any further questions.