But wait why does he say son in law when he knows that’s his son...... I’m confusion

i dont know how to explain it but it’s not about wo wo. a son in law is a husband of one’s daughter or son. it’s usually what you will be called by the parents of your partner once you get married. for example, if you’re out with the dad of your wife, he will introduce you to people as “oh hey, this is my son in law” haha.

The idea is that he calls himself son-in-law in the sense of wife's husband. So he's calling Ci Nian his wife, which is why he says not to let him hear it
(=・ω・=)