As he crashes into the lake, Brian has basically no knowledge of the wilderness. He had flown over it previously to visit his father but never spent any significant time in it. The fact that his mother had given him the hatchet was somewhat odd in light of this fact, but obviously it was vital to his survival.
So Brian gains his knowledge through painful experience. From his very first day getting bitten ferociously by the mosquitoes, he begins to stockpile understanding and experience that will serve him well. He learns not to eat just any berries after the gut cherries give him horrible stomach pains and other awful symptoms. He learns that he has to keep any food in a safe place after dealing with the skunk that stole his turtle eggs.
Eventually Brian feels like he has gained enough knowledge that he is confident in his existence there at the side of the lake. He is obviously happy to be rescued when the bush pilot lands on the lake but he is so comfortable that, rather than race out to get on the plane and get away, he invites the pilot to join him to eat the meal he's just cooked.