How do I break the news to my family of very bad medical news?

Received a call from the doctor this morning that the cancer we put into remission 4 years ago is alive and well and has spread very aggressively. When I asked about my mortality, all he could say was ‘We’ll buy you as much time as we can’. I have not told my family the extent of my illness. I have come to a point that I have to tell my wife and son more about the fight I (we) are about undertake. How do I tell them?