Re-reading this thread reminds me of when the doctor gave the prognosis that I probably have breast cancer. I went home to my 10-year old, at the time, and was totally honest with her concerning it. And she said something that changed everything for me. She said "Well, mommy, we are just going to treat this as an experience. You know we like experiences, even some bad ones." Those words just reset my mindset, and I went through the entire process as something new and interesting, and that made an entire difference (and still does).Kids can be more mature than we think at times.
Of course the other option is to sit down with him and ask HIM what he wants to know - how much etc. Show him you trust him and want to share - but only if he wants to know. Again, that gives him some choice over how much he wants to know, and you don't have to worry so much about scaring him or something if he really wants to know.