The River by Gary Paulsen

Two years ago, Brian Robeson was stranded alone in the wilderness for fifty-four days with nothing but a small hatchet. He survived. Now the government wants him to do it again-to go back to the wilderness so that  astronauts and the military can learn the survival techniques that kept him alive. But this time he won't be alone a your government psychologist will be coming with him. But who knows if the will come back alive.