The magic mouthwash is given to radiation patient to help numb the pain from the blisters in the mouth and the sore throat. Different hospitals have slightly different formulations but they all serve the same purpose.
You can get him a Therabite to help him exercise opening his
mouth. I know some people use stacked popsicle sticks. There are also people who have tried the Therabite but don’t think it’s worth paying so much for it. You can do a search on the forum. There’s been a number of posts on it.
I understand that the chemo acts as a radiation sensitiser and helps the radiation work better. There are different kinds of meds that patients are given to help them deal with the nausea and the discomfort. If your dad is seeing the medical oncologist, make sure you talk to the doctor about your worries and see what the doctor’s opinion is. I’ve found that being open with the doctors has advantages to it.