Your father is an elderly diabetic with a history of longstanding diabetes
He is have kidney related complications also.
may have neurological side effects but you should see the larger picture.
Due to this kidney problems it may be due to electrolyte imbalance
Diabetics are at a higher risk of development of brain infarct and stroke.
Brain degeneration is a natural phenomena in old age and diabetes will accelerate the process.
Consult a neurologist