Hello Rolanda
His problems do seem to be long lasting.
Has he been prescibed the Magic Mouth wash, that helps a little. Or there is another drug called Gelclair a slightly newer and more expensive mouthwash? A lot of time the lack of moisture in the mouth can add to the blistering and discomfort , Salivert a spray moisturiser might help there?
Have him try and take this drink , lots of calories, 1350, and easy to swallow.

One package Carnation Instant breakfast
2 tablespoons Ovaltine
1 tablespoon Carnation powder
1/2 cup whole milk
1/2 cup half and half
2 cups ice cream.
I am surmising from your post that he does not have a feeding tube?
You Doctors telling him "he should " be eating better is of no help ...kick up a fuss till his oncologist listens to you , and if he needs it get him an anti depressant. Dont be passive , it doesn`t work with Doctors.
Let us know how you are doing , we care .
Marica