A Korean drama with a mentally disabled main lead ??

You should try "Birth Secret" that is on you plan to watch. The male lead is slightly mental disable and the female lead is a genius. I would recommend "Herb" a korean movie with a disable female lead.

There is a mentally disabled father in Can you hear my heart? but he's not the main character but he has a big role

The Hours of My Life is a new Japanese Drama
Plot:
Takuto Sawada (Haruma Miura) is an ordinary university student. One day, he learns that he has ALS (Lou Gehrig’s disease) and doesn't have much time left to live. Instead of wallowing in despair , Takuto tries to live his life actively.

The Korean Movie Secretly, Greatly the main pretends to but is really a spy
http://asianwiki.com/Secretly_Greatly

those are the only 2 that come to mind sorry not sure if either are what you are looking for.

Good Doctor's summary sounds like what you're looking for though I haven't watched it myself. A Moment to Remember also features a lead who becomes mentally disabled.