It probably took a lot for him to even just tell you.
Since I don't know him, I'll tell you what I wanted when my mother was sick and even more so when she passed away: I wanted people to be there for me. I wasn't only losing my Mom, I was losing my entire family and that hit me hard.
The best you can do is help him (with homework, chores), talk to him (make him laugh, that's so important), take him out to the movies or something you would both find fun.
Even just doing homework (if you're in school) in the same room can take a little edge off and may encourage him to keep up with it more.
But it's hard to say. I was an 8 year old girl when this happened to me so what would've helped me might not help him. But good luck and I'm sorry you're going through that stuff.