I know about the pain that you are talking about. As mentioned before-cream on tissue between the cheeks really helps for me. Maybe you could purchase a very small kiddie pool for a sitz bath?
been there done that. i used every cream you can imagine. the one that worked for me is. Utter balm. you can find it at Walmart. They use this on the cows teets, when they are cracked and dry. it was a god send for me. good luck. hope all works out your granddad.
Treat people the way you want to be treated. Simple.