John learns that he's terminally ill and is going to die. He takes is as a doctor and a soldier - as stoically as possible. But he knows Sherlock is going to take it much worse. So John starts taking care of everything and everyone he's going to leave behind. Maybe he helps Harry somehow. And he also talks to Mycroft and Greg, making sure they will look after Sherlock after his death and will do everything in their power to prevent him destroying himself with drugs and other things.