Hi Kee,
With much emotion, I know exactly where you are at.
When I was diagnosed in 2007, my youngest daughter was 5. It was such an emotional time for us all, but more so her. THere's no parent manual at the best of times, let alone for a cancer diagnosis!
Firstly, if you can get it, get a copy of My Mum Has Breast Cancer! AMAZING!!! The Author, Lisa Seward is also a member of BCNA and she wrote it along with her son Harrison who was 6 at the time. Its fan-bloody-tastic! The illustrations are all by her son. I cant recommend it highly enough. Here's the link: http://www.bcna.org.au/news/information-resources/reviews-books-and-other-resources/books-children/my-mum-has-breast-cancer
I ended up taking my daughter to a Psychologist (under the Mental Health Plan) to talk through some things and to help her understand. This was AFTER treatment, so she was about 7. It really helped.
She still gets angry with me and I find our relationship has "changed" since diagnosis. She seems to oush me away alot but then gets really upset if I'm not "there for her". Its a tough time but the best thing I can offer for you is, a) get the book and b) be flexible with his emotions and keep him informed with ALL of what you're doing (age appropriate, of course) treatment wise.
Good luck and lots of hugs!
Sam x