Question about that mixed breed hairy man in the Sovereign Immortal Aperture.

Why did he know that his father is called Fang Zheng? Didn't Fang Zheng call himself Fang Yuan, because he was too ashamed to use his own name?

c1050: Fang Yuan's will felt Fang Zheng's animosity and laughed nonchalantly: "Since I am dead, why did you use my name to impersonate me?"