Have your father checked to determine if ther are calcus (stones) deposits in his salivary glands. When I worked in Iran I started to have similar effects. I was diagnosed as having a large stone in a sub maxillary salivary gland on the left side of my jaw. I had surgery and they removed the gland. It did not make any difference as at age almost 70 I can still drool a lot.
Perhaps your dad has the onset of the stones and the glands are blocked. If this is the case the Doctor may prescribe drinking of distilled water that has no minerals in it.
------------------
The Cat