I was the child in this situation 13 years ago. When I was 12 yrs old, my mother passed away from cancer. My father took us to see my mom at the hospital on the days that she was up for it, but at the end of the day it was her choice. My mother didn't want us to remember her as a sick woman, but the woman who loved and raised us, who took us on vacation and to baseball games.

Needless to say, I understood that she was sick and thought she would recover, but unfortunately it wasn't in the cards. I came home from grade school one day and my dad told my brother and I that my mother had passed. It was devastating.

OP, there is no right way to address this, the situation is miserable and the only thing you can do is make the most of your time left together. So what you need to do is make sure that your son is ok. Make sure he knows that his mother loves him. Make sure they get to have some semblance of a good bye. And make sure, when the time comes, that you don't stop talking about her - keep the memories alive, because it does help. In a few years he won't be as sad when he thinks about his mother. As humans we tend to remember the good things. So it's your job to make sure that you remind him about his mother and all the great things she did for him, even if it was something as simple as packing his lunch before school or working the school fair after a long day of work.

You guys will get through this. You're in my thoughts.