First time around I had radiation to the tongue, neck, and about 4 weeks into treatment just about everything tasted like sawdust. Later, couldn't eat anything because of the pain and wound up, like your father, in the hospital for feeding and re-hydration.

The taste should come back to a certain extent later with time. Probably some things won't taste the same for a long time. If you look back in the forums, a lot of people loose the abilities to taste sweets and chocolate, others spices, just depends on the individual.

Thing is to explore different foods and possibly cuisines that you father might like with his changed set of taste buds. Seems I don't like a lot of things I used to love, but now like a lot of things I wouldn't touch before. Go figure.
Bob