It's not the pond as much as it is Holden's concern for what happens to the ducks that indicates his concern for himself and his own fate. Each time he questions what happens to the ducks when the pond freezes over, it demonstrates his anxiety about what his future holds.
Remember, Holden is by himself in NYC because he has just been expelled from another prep school. His parents haven't found out about his latest academic issues, and he is worried about their reaction. We know that Holden doesn't have a strong relationship with his parents, and the only family member he seems to like at all is his sister Phoebe.
Holden is aware that he doesn't fit in with society's norm, and he has a sense, albeit a subconscious one at this point, that he is arriving at a turning point in his life.