Cheryl,
My husband had radiation and chemo. The mouth was very sore but he was able to get through it. They should tell you to use a mouth rinse made up of quart of water 1 teaspoon baking sode and 1 teaspoon salt. This will help with the mouth sores. You can make this up yourself. It is expensive to have the pharmacy make it up. They told us to rinse at least four times a day and then increase it as the weeks go by. My husband would use it every time he spit and in between spitting. He probably went through at least two quarts a day. He also used it during the night if he had to spit.

I wish your father well. He can get through this.