If my son served as an Army MP at Camp Humphreys in South Korea for a year, does he receive the KDSM?

I served up near the DMZ in 1970 and received the Armed Forces Expeditionary Medal. Years later when the Korea Defense Service Medal replace the AFEM, an exception to policy allowed bit award to be worn SGT Larry Pearson

Yes, you're entitled to the KDSM after 30 days in country.

30 days consecutive or 60 days cumulative.

Reservists can usually get it after three AT exercises over there, and most certainly 4 exercises.