Brian simply means that feeling sorry for himself isn't going to get him out of his current predicament. He's in a desperate situation, having just crash-landed in the middle of a forest. This would be a scary situation for most adults, let alone a teenage boy with absolutely no experience of surviving out in the wilderness.
Brian's natural reaction, like most people, is to lament his fate, but self-pity quickly turns to resolve. Brian doesn't have time to sit around feeling sorry for himself; he's got to think of the practicalities involved in living off the land. That means finding food, shelter, a regular supply of fresh water, and so on. Self-pity may be the natural reaction to someone in Brian's situation, but it won't do anything to help him survive or to help him overcome the enormous challenges ahead.