He is definitely not ASPD, neither socio nor psychopath. Neither primary or secondary psychopathy. 

As for secondary psychopathy, sure the childhood trauma took a huge toll on him but it never really ended up as ASPD.

What we can see is that he was  unable to distinguish his emotions and empathy, which he felt a lot of . so is this like  a form of autism? alexithymia? 

I was thinking the same thing while watching the show. I feel like he likely has CPTSD, and then maybe Autism (ASD) and/or Borderline (BPD). I was disappointed they didn't have someone fight for him to be re-evaluated when he was clearly misdiagnosed with ASPD (although I LOVED his wife screaming at the doctor for misdiagnosing and writing him off), but I was impressed with the sensitivity we already got since Korean media doesn't have the best track record when it comes to discussing/depicting mental health issues

He is traumatized and trauma can numb emotions and empathy 

I think he had trauma and depression from his mother leaving, but he got misdiagnosed. Because of this he started to believe the diagnosis and truly lost most of his feelings. Later on, by faking them he slowly started to feel things for real. I think he was his own greatest enemy. Because he truly believed he couldn't feel, he also didn't (or didn't let himself). I think he wasn't actually very seriously mentally ill, except for depression, trauma, self hatred and so on. 

Oh, and he saw his father, that wasn't really there for a while...maybe some schizophrenia or psychosis?