Is your dads loosing weight the only symptom, or does he have any other problems?
I am asking, because my MIL has started loosing alot of weight in her late seventies, now at age 85 she is skinny, but healthy. The doctors could not find any reason for her weight loss, they checked everything, she is fine. Actually she is in better shape than I am.LOL
So maybe it is just your dads age.
I may have been dealt a bad hand, but at least I'm still playing with a full deck. ( most of the time anyway).