Rob's antagonist isn't really another person. There's no one character who's standing in his way, keeping him from getting where he needs to go or holding him back from reaching a goal. Unfortunately, the forces he's up against are a lot bigger than that. The passage of time, the grinding poverty that brings his family so close to the edge that they have to sacrifice Pinky in order to survive, the unidentified sickness that kills his father—all of these things function as Rob's antagonists in the story.
If you want to get super deep, though, though, you might even say that it's Rob himself, and his own childhood, that he has to overcome in order to take up the mantle of manhood and learn to do what he needs to do.