siamputhomte,
I am sorry to hear that your Dad is having such a rough time, but radiation and chemo tend to have these affects on our mouth and throat.
Was your Dad offered any pain medicines for any discomfort with his mouth sores? If not, ask for them now.
For a short term solution to the problem, I was told to use a solution of 1 teaspoon of baking soda and 1 teaspoon of salt in 1 quart of room temperature water, rinse and gargle as needed. I used it just before eating to numb my mouth a bit. I increased the mixture to 1 Tablespoon of each in the later weeks of radiation and it helped, but eventually, I needed the pain med's to help also.
Roselvr got a perscription for Actiq-pops for her husband. They are like Lollipops and he used them during the day to numb his mouth. You can see about those also.
My Best to Both of You and Everyone Here