If a Pakistani Citizen gets South Korean Citizenship, does he has to lose his homeland citizenship?

He would automatically lose his Pakistani citizenship when naturalizing in South Korea, as Pakistan only allows dual citizenship with 19 countries, and South Korea isn't one of them.

Even if that were not the case, South Korea requires renunciation of the old citizenship within a year from anyone who  naturalizes in South Korea.

So it's a double whopper.

Korea generally does not grant citizenship to foreigners, so it is highly unlikely you can obtain citizenship there. Pakistan apparently does not have mandatory revocation of their citizenship if you obtain another citizenship elsewhere, but that does not appear to be much of a concern in this case anyway.