In terms of Brian laughing at his own description of himself as "city boy," I think it goes to show how much Brian has changed. At this particular point in the novel, Brian has developed his own sense of self to a point where he understands how different he has become as a result of his time in the wilderness. When Brian reflects on himself as a "city boy," he senses that the person he was before his experience is vastly different than the person he is as a consequence of it. For example, before the experience in the wildnerness, Brian used to waste food and not conserve anything. He also took food for granted. However, as a result of his experience, he has become more mindful of conserving and saving anything and everything. There can be no waste of resources because one never knows what will be needed in the future. The remark of self as a "city boy" reflects from where to where Brian has come.
Brian laughs at this because his transformation as a human being has dramatically changed. As mentioned in the above answer before me, Brian used to be a "city boy" because of his history and experience within the city. He would waste food, and not take full advantage of the resources around him. This is complete opposite thinking that he had in the wilderness. Instead of being given food and taking it for granted, Brian now had to earn everything, in order to survive. This type of experience that he went through, would be very life changing for any human being. Having to be placed in the complete opposite type of atmosphere, such as the one Brian was placed in, forces you to adapt and step outside your comfort zone. For Brian, this was a necessity for his survival, and when he laughed at the "city boy" reference, it put into perspective how much he really had changed.