Yeah, unfortunately it sounds like your only two viable options are either to schedule a 1-on-1 "disciplinary chat" with the kid, or to force the issue with your co-teacher and make HIM deal with the kid. Of the two, I'd recommend the chat first, and the co-teacher intervention second if it's still necessary.
I think flasyb has the right idea about how to conduct the disciplinary chat. Keep the kid for long enough for it to get uncomfortable for him, and force some genuine answers out of him. Lots of eye contact, no anger, and lots of concern. Most of the kids who act like this are not used to thinking about their victims as people -- it makes them very uncomfortable to do so, because they don't want to think of themselves as being mean. Once he sees you as a person, things may change for the better.
If he's still acting like a monster after that, you WILL have to get your co-teacher involved, because by ignoring his actions, your co-teacher is tacitly supporting the student's rudeness. Guaranteed the student is aware of this, and that's why he feels comfortable blatantly refusing to do what you say. For this co-teacher to tell your main co-teacher that nothing is wrong is further proof that this guy has no respect for you. Unfortunately, confrontations like the one you will have to have with him can really come back to bite you in Korea, but if the alternative is having this kid disrupt class every day, you don't have much choice.
Good luck!