I totally agree with all of the above. Microscopic or not this cancer can be aggressive, as you may have noticed. He should complete all treatments. Ideally he should also be able to eat and drink. I am assuming at this point he does not have a PEG tube (G-tube) to accept feeds. I think they may not choose to do this for him at this point (they may - I would ask). However if they choose not to you could ask for an NG tube to get him through the next 4 or 5 weeks. This way he would be able to maintain his nutrition and fluids. This would go a long way to solving his main problem which is going to be malnutrition, and dehydration... (sodium level of course is related to this.)

For his mouth. Ask the dr. for pain meds, snd a prescription for Magic mouthwash (its a rinse containing a few different drugs - often lydocaine is one of them - its a numbing agent which temporarily numbs the area so he can drink and swallow) and invest in some boost or ensure (whatever the highest calorie/highest protein nutritional drink available in your area is. Here we have resource. Try to get him to take a bit by mouth, and the rest by tube.

He needs to rinse after every meal/drink and in between if he can. There is a baking soda, water, salt mixture that most of us used throughout treatment that you can make at home. He can rinse and spit this. This, believe it or not, keeps his blisters clean and any sores as well and helps heal. (flattened club soda will also do the trick and he can drink the club soda which will help soothe his throat.

Skin issues. Cream. A good solid cream - there are a few here that are fantastic. I am sure you have some there... shea butter? something to soothe the area. do not apply it before his radiation. Put it on right after.

Is he receiving chemo? Often they give it in conjunction with radiation.

I really would advise you to push him to finish. You should have been told what to expect. It's not a fun treatment but it works.

Take care and have a great day.