Leaving his dad behind was not a hard choice. Bradley was never really blinded by that family loyalty that his older sister had—but maybe that was in part due to the fact that Bradley had only barely turned two years old the first time their dad had left.
When he first tried to understand the situation, Bradley had taken to pitying the man—believing that there was something fundamentally wrong with the way his dad processed the world, or processed happiness or whatever and just couldn’t help himself with his serial flitting in and out of their lives.
But then Bradley learned about his father’s other families, that his mom wasn’t the first or last woman he’d done this to—and his pity was gone, replaced with the pure and simple disgust. He couldn’t even be bothered to hate his father because that would take too much of his time and energy, and it would have about as much effect as trying to clean his shoe by hating the dog turd he just stepped in.
So, he just left the man behind, because why bother?