The primary tool that Brian uses throughout the novel is the hatchet.  It sounds silly, but once the pilot teaches him how to fly the plane, he refers to the hatchet many times.  It is what helps him survive in the wilderness, protect him from intruding animals, and becomes invaluable.  In a larger sense, one of the things that Brian gains is that he uses as many elements as possible as tools for his survival.  For example, he uses the hollowed out cave as a shelter.  In finding sources of food, he uses his surroundings to help feed his initial hunger, and then develops a more intricate use of his surroundings to gain greater and more substantial food sources.  While the hatchet is the tool primarily used, Brian develops in his survival by using many more tools as critical elements in order to endure and eventually triumph over his wilderness surroundings.