all old kitties needs love and care!
it is difficult to say what could be wrong without a physical examination, I do not know his neurological examination, nor do I see how he walks.
With the information you gave me and knowing kidney function tests were done, I would think about doing more tests like: a urine analysis, a full blood panel with a thyroid level and some radiographs whole body. It is not clear without an examination if you have weakness or a skeletal/spinal problem. Skeletal problem (like arthritis) are painful, mind you kitties are tough and will hide all signs of pain until they can't hide them anymore. This is a trait in feline necessary for their survival over the years.
Cats are prone to develop heart disease as they get older, hyperthyroidism, arthritis in their hips/knees/back. Cancer is also a possibility.
Hope this helps.
Keep me posted.
In the meanwhile keep him indoors since he cannot get away as fast as he can if their is a problem. If Patch goes out, be with him to make sure he is safe.
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Hope this helps, keep me posted.
If you think of anything else