Obviously he didn’t know it. Taekyung himself never told him nor their mother. Teakyung didn’t even try telling him about their mother actually, and who in their normal mind would suspect that their mother could do such an act? Obviously if he did and I’m wrong then sorry.
I mean Heeseo is right u didn't know anything about Taekyung...u didn't know fuck bout what ur mom did to him obviously...fuck