Margaret,
I would think that if you are miserable you would automatically try almost anything to make it better....
How about this: just prepare a bottle of the salt/soda mixture and let him try when and if he is ready. It is certainly a lot more comfortable than rinsing with water. I use this to rinse my mouth after brushing teeth too. When you make it, taste it and see that is does not taste too salty for you.

Also, watch out for infections, especially post radiation when you see the RO less often. These can make you feel miserable too and cause difficulties in swallowing, speaking etc... I had BOTH the RO and MO look at my mouth and both figured that this is as it should be at this stage... Well, I also had a slight fever. I took diflucan and within 3 days the fever etc was gone and I can even swallow normally. I still have mucositis of course.
A note of caution, you should not take prescription drugs without the consent of an MD. I wonder why neither RO nor MO came up with any of this? Maybe one also needs a GP with some common sense!

M