When my husband, Mike was at your son's stage of radiation, the weekend "break" did allow him to feel better. The Magic Mouthwash initially helped some and then it didn't, then a few weeks later helped again. So, don't give up on a product, it might help later.
Anne- Marie, I sympathize with you and can see your dilemma in wanting to help your son and being the Mom. I've got 2 grown sons and struggle with that enabling issue all the time, and they are not sick! It's so hard and no matter what you do, it haunts you that you've made the wrong decision on independence vs enabling.
However, in this situation, with your son being sick, I would vote for helping as much as you and him can handle. The hardest part for you will be dealing with the fact that sometimes you cannot "fix it". You may hear the same complaint from your son over and over, day after day. You could drive yourself ragged trying to get something to work. It took me a long time to realize that really what my husband wanted was someone who would listen to him.