cjack,
All of these wonderful posters are giving you great info. Unfortunatly, what works for one person doesn't always work for another.
Radiation damages the saliva glands. Saliva is composed of two types of secretions serous and mucous. Radiation wastes the serous secreting component of the glands worse than the mucous part. So, saliva gets thicker and more sticky "phlegm like" and less runny and "water like".

Keeping hydrated is helpful, using a humidifier in the room he sleeps in may help, keeping some water by the bedside may help, drinking water prior to trying to eat will help lubricate the mouth, lemon drops can help stimulate saliva flow (lemon may hurt the sores), drugs like salagen help some people, and saliva substitutes can also help.

Mouth sores are nasty and hurt a lot. Magic mouthwash is good. Biotene has an enzymatic nouthrinse that helps many of my patients. Using toothpaste that does not contain "SLS" can reduce the incidence of mouth sores as SLS can encourage sores to appear. Closys toothpaste (available at Walgreens) does not contain SLS and there are other toothpastes available that do not contain SLS ask your pharmacist. (SLS is sometimes on the label as sodium lauryl sulfate, or SDS sodium dodecyl sulfate)

"Thrush" is a fungus, candida albicans, that is part of the normal flora of the mouth and can be an opportunistic infection that occurs when the conditions are right. This needs to be treated with antifugals such as nystatin oral suspension or diflucan.

Saliva glands can regenerate in some people.
Excessive phlegm, mouth sores and thrush all at the same time is a monumental problem but, it can be overcome with proper treatment and perserverance. Talk to your Docs.
I hope this helps a little.
Cheers,
Mike