So sorry to read of your friends news.
It's always a shock to get news of something like this with children, and nothing anyone can say will make things right.
However, my wife works as a volunteer at a childrens hospice, and I do have some experience with leukemia and other childrens illnesses.
The thing that we see surprisingly frequently is that families with children who have life threatening conditions like leukemia often find friends and acquaintances steer clear as they do not know what to say, and giving a wide berth is easier than approaching someone and not knowing what to say.
My thought is to be there when your friend needs you, and over the long term, there will be difficult times when chemo or other therapies are used and this is when your friend will need support the most.
Your friend will often feel isolated, as his priorities have just been changed without warning, his values have gone out of the window, and he will have to adjust to a new normal of hospital visits, doctors, and nurses and supporting his family in a way he never imagined.
A whole new language will have to be quickly learned, and if you can, try to learn some of this language so your friend can communicate with you on the same level.
You and your friend will get a lot of help and support from this site, there are lots of uplifting stories, and it can give advice on how to live with leukemia :
http://www.lls.org/
If there is anything practical I can do to help, let me know.....