The United States, or even the UN, has NEVER signed a peace treaty with North Korea, so technically, we are in a war that never ended, even with the armistice of July 27, 1953, and that only happened because Eisenhower, then the new sheriff in town, made a not so veiled threat to use nuclear arms to end it.
All this happened long before Kim Jong-Un was ever born. Perhaps he should read up on a little history and find out why Grandfather Kim never went any further.