Dear Gramma Elaine,
Can't really help in terms of what you are asking, but you need to think in terms of how your Dad's teeth are being cared for. If he is in reidential care, check that his teeth are being cared for by someone who knows what they are doing. So often cleaning of teeth is overlooked, and if someone on the staff is assisting your father, find out what skills they have.
Ask someone to clean your teeth for you and see how it feels!
Overzealous use of a toothbrush by someone who either doesn't know how . . . . . or doesn't care . . . . . can damage teeth, fillings, and the inside of the mouth.
If there are mercury problems this could make the whole thing worse.
Over years of visiting care facilities, this is one thing which scares me a little. Often the problem is ignorance.
So if you do nothing else, review the care he is getting.
Good luck.