excessive mucus production as a result of radiation for throat cancer

I am writing this for my 79 year old father because he has a hernia and cannot sit for long.
He had radiation which successfully stopped (inadequate word, I know) his throat cancer in 2000. Immediately after the radiation, his body started producing excessive amounts of mucus, which, along with the scarring (affecting the swallowing), has prevented him from eating for the past five years. The swelling in his throat grew so bad that in 2004, he stopped being able to speak.
In March, 2005, he had his voicebox removed, and was told that he should be able to speak (with a prosthesis) and swallow again. He was also told that the amount of mucus he produced was "normal" but that he couldn't swallow it.
Since the operation, the mucus production has increased, and it has gotten thicker. He has difficulty sleeping and lying down. They can't insert the prosthesis yet because the fistula has a leak (common in this type of operation, we're told) which will require further treatment. And he got a hernia from coughing up the mucus.

What he wants to know is:
1. The doctors have said all along that the mucus production is a "normal" side effect of radiation. Why does radiation produce this affect?
2. If the site that was radiated (the necrotic tissue) was removed, why is the mucus production getting worse?
3. Are there other throat cancer survivors who deal with this over a long period of time? Most of the literature talks about dry mouth, but not this drowning in mucus.
4. Does anyone know of any effective treatment for it? Any medication?

At this point, my father is very depressed. Each procedure has been described as promising, and yet he still can't eat, can't speak yet, and the mucus keeps coming. Frankly, the doctors seem stumped by what's happening to him.

Any and all answers would be greatly appreciated.