Yes. That is a ROK army soldier who is part of the UN security force at the Joint Security Area in Panmunjon, which is in Korea, but technically is not in any country. It is in exact center of the DMZ. Their are also American guards. They are probably some of the toughest and best trained (for their limited mission) troops in the theater.
In this case, the American and Korean forces are still in Korea as part of the UN mission started in 1950. The troops are all under the command of the UN, but the commander for the theater is an American 4-star, who also commands the United States Forces, Korea, and the 8th US Army. Panmunjon is a "truce village" where there are ongoing talks about the war. The Korean War never ended, see, there was just a cease fire, which occasionally gets broken. Representatives of the various countries involved (the US, China, North Korea and South Korea, all meet at Panmunjon to for ongoing "talks" about the cease fire.
Panmunjon is probably one of the most tense places on earth. There is an actual yellow line (the Military Demarcation Line) painted down the center of the village. Armed NKs are on one side, and Americans and ROKs (like the guy in the photo) are on the other, in a perpetual high-noon standoff, waiting for gunfire to break out, which it periodically does. This ROK isn't "taking a whiz," he is using the side of a building for cover and waiting for a chance to bag him a North Korean. I personally wish him, and his American buddies good luck with their endeavor.
As far as the UN goes, the UN gets lip service, only, and has no direct control over anyone's troops , although the mission in Korea is technically a UN mission.