In the epilogue, the narrator tells us that Brian had spent fifty-four days alone on the lake before being rescued.
The experience changes him in many ways. Um… duh.
Physically, he's leaner and tougher than he was before. He's also far more observant, and far more thoughtful. He no longer takes any food for granted, and he continues to be amazed by the plentiful and abundant resources that are available to him back in his ordinary life. (Didactic, much?)
Brian also does research to learn more about the animals and plants in the woods. The food that he called "gut cherries" are known as chokecherries, and are commonly used to make jam. The "foolbirds" are ruffed grouse. The fish he ate were bluegills, sunfish, and perch. That's more than we learned in science class.
Brian has dreams about the lake and the woods. They aren't bad dreams, just vivid and realistic dreams of his life on the lake.
If Brian hadn't been rescued when he was, the narrator tells us, he would have had a very rough time in the woods when winter came. The lake would freeze, making the fish inaccessible, and prey would become scarce.
Despite a brief period of family unity and happiness when Brian is first rescued, Brian's family situation quickly returns to what it was before the crash. His father goes back to work in the oil fields and his mother keeps seeing the man in the station wagon.
Brian is never able to tell his father about the man or tell him the secret about his mother's affair.