When Brian grows desperate for food, he thinks of his teacher who used to tell him that he was his biggest asset in any situation. Then he began to consider that there had to be berries somewhere. He eventually found some and even though they were bitter, he ate enough of them to fill his belly. Later that night, they made him terribly sick and he began to call them "gut cherries" from then on.
Later he found other, sweeter berries and these didn't make him so sick. The next thing he found to eat was turtle eggs, which he ate raw. These were significantly more nourishing and helped him gain some more energy.
The great discovery he makes several days later is that of remembering that water refracts light so once he leaves the tip of his arrow in the water, he finds himself able to get fish with it and he finally sates his hunger with all the fish he can cook on the fire.
He also goes on to figure out how to kill birds and he takes great pride in that kill, the day of first meat. The last thing he finds is of course the survival pack in the plane and he has all kinds of food ready to eat when the bush pilot lands on the lake to rescue him.