Gnu, don't be SURE he's dying. Sometimes the chemo kicks a person's ass harder than the cancer.

In any event, I'd say just fall back on the friendship. There will be people who want to avoid the subject, and there will be people who expect him to spill his guts. Don't be either--just let him be whatever he needs to be. You guys are buddies and that's all you have to be. Cancer is a big, big shadow. Even if it will eventually get him, there's a lot more to life NOW. Keep making him laugh.