Oh Trazzle, it is so awful what both of you are going through. It must make it even harder that you can't even physically comfort him, the poor poor boy.
As others already have said, and it is also my experience with dd, Joe will later forget all about this period in his life and unless you have photos/hospital bracelet etc. he will not being able to remember any of this at all. Unfortunately, at least in my experience, it will stay with you for the rest of your life. 
I suppose the only thing left is to be there so he knows he is not going through this on his own but got you there with him. If he likes a story, lovely, but if even that is too much, then just being there is all you can do.
Has the hospital said anything about where he is at the moment, eg do they expect him to get even worse or do they expect him to start improving soon?
I am so sorry Joe has to go through all this.
Gingers massively crossed that progress is made and alternative, less awful neuroblastoma treatments are developed.
Thinking of you both a lot,
x