Radiation to the H&N is not without side effects. If he finished 6/22 though, he should be long over any ulcerations by now. Why hasn't the RO treated him for this? Liquid Vicodin, Miracle mouthwash, whatever, etc., it can and should be dealt with.
Dry mouth is the new normal, get used to it. It may get better over time if the RO had the ability to spare a parotid saliva gland, but that will take a year, at least.
Things will not be the way they were, ever.