Your father is very old and it could be just his age. His medications could be causing dry mouth as they usually do.
His abdominal pain should be addressed by a doctor to see what could be causing it. It could be some kind of nerve problems since the pain is radiating and if the pain is severe and radiating down the groin or upper leg, it could be a kidney stone.
You should take him to a doctor for an evaluation. Not much can be said without proper examination although symptoms are very good indicators. He should explain to a doctor when the symptoms started, if anything triggered them, if anything aggravates them and anything there is to be said regarding the pains.
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