Hi Laurie; A dry mouth and altered acid base balance can turn our mouth into quite a garden (not the nice kind.) Frequent swishes with a soda saline solution...every two hours during the early weeks and of the use of the Biotene products are very helpful.
I would be tempted to have the doc look at his mouth as he may have a fungal infection tht can be quickly and easily treated.
Dental hygiene is paramount. With no saliva to swirl away the trapped foods things can easily get lodged between teeth. Flossing and brushing after every meal have become part of my routine.
Some of us have taken to "power tools" like electric toothbrushes and water-pics and for the most part I think they are really helpful. I am determined to keep my teeth so I am highly motivated.
Ten months post rads I have a tiny amount of saliva...not enough to make a significant difference in eating. It seems like we all have unique reactions and recovery times.
Hope your brother has a speedy return to comfort.
All the best,
Fran