Friend, when we see acute onset of loss of motor function to the back legs this usually occurs with disorders affecting either the vertebral column (spinal cord) or hips. Common possible causes are: Arthritis of the hips, spine, a slipped disc, tumors, etc. So, it is very likely that this could be related to severe arthritis of the hips/spine or both.
My other concern is that he is drinking more than usual. When a dog drinks more water than usual it is a sign an underlying medical problem. There are many possible causes. It will be difficult for me to determine the exact underlying cause without a hands on examination and some testing. But, common causes are: An active infection, hormonal disorders (diabetes), kidney or liver disease and even cancer to name a few causes. Can any of these diseases also affect the motor function in the legs? Yes especially diabetes or even tumors (cancer) in the abdomen or limbs.
In this case unfortunately we are very limited as to what you can do at home. I would strongly recommend having him checked first thing in the morning by family vet for blood work (chemistry profile, T4 & CBC), xrays of the hips/spine/abdomen and urinalysis as initial screening test. Treatment will be determined by clinical and diagnostic findings. If this is related to arthritis he will be started on prescription anti inflammatory medication and pain killers. Unfortunately not much I can recommend for you to give over the counter. Do Not give things like aspirin, tylenol, advil, etc. until he is seen by family vet and has had blood work done.
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