Uhm this is assumption but since he got sick they called for the doctor and changed his clothes so assuming the cut scene where the doctor called for mom and mom is shocked, its the doctor who figured out that the cause of fever is that he slept with a man, like he didn't get proper rest after doing the deed and knowing he has a weak body that's probably cause the fever. ╮(￣▽￣)╭
For me, its because everytime he got sick they call a personal doctor or rush him to the hospital for over all testing and maybe when the doctor check on him they find his markings on his body and his hemorrhoids.
someone explain to me how did Yuan's mom find out he slept with a man? she found out he's had tattoo ok makes sense but how did she know he slept with a man? it's been bugging me