I'm sorry for your troubles, and welcome to OCF. Your father's behavior sounds like many other cancer patients going through or have completed treatments, although, any pain should be mentioned to the doctor to rule out a new cancer, secondary or remaining cancer. I don't know what pain meds your father is on, but check the WHO Pain Ladder, and discuss them with his doctor. If I get a chance, I'll post later. There is no reason for him to be in pain, if pain medications, anti-inflammatory, nerve meds, can be of a relief. Other conditions cause pain too, so it's probably multifactoral, like mucocitis, thrush, inflammation, ulcers, dry mouth, other infections, which need to be treated appropriately, impeccable oral care, rinses, foods that are not hot, spicy, acidic, sharp textured.

Good luck with everything.