My answer would be that stage 3 and even stage 4 cancers are absolutely curable, especially if you look first to nature instead of mainstream treatments. Mainstream medicine has but I would caution that mainstream treatment methods by and large fail to cure anything since all they do is attempt to cut out, burn out or poison out the symptoms of cancer instead of addressing the underlying causes that enabled cancer to defeat the immune system and gain a foothold in the first place.
I am not advocating rejecting mainstream medicine so much as saying to educate yourself and see what nature has to offer, The truth is far, far different than what we have been told and conditioned to believe.
Here is a good place to start:
Modern Medicine versus Nature in Treating Cancer
http://www.tbyil.com/MMvsNature.htm
My background is this: I have researched and written about natural alternatives in treating cancer as well as natural health in general for many years and I host a non-commercial internet health group on Yahoo (not plugging it here) where there are hundreds of members and almost 100% success in beating cancer, even at late stages and even after mainstream surgery, chemo and radiation have failed.
Whatever you and your loved one decide is best, I wish he and you a long, healthy and happy life.