Hi Brandi,
My husband's neck and mouth burned badly enough that they suspended his treatment after his 26th dose of radiation on March 20. They plan on resuming treatment on April 1st. His neck actually blistered up in 2 large areas. His skin might be more sensitive than most due to incurring severe burns to 53% of his body 19 years ago. About 4 weeks in Jeff started having the same problems as your dad, including the mucus and difficulty swallowing and speaking. The doctors had warned us that this would happen and that it's absolutely normal. They're treating aggressively to make sure they get it all. Your dads treatment sounds very similar to my husbands and I know how hard it is to see. Hang in there!