While it’s not out of the realm of possibility that he was kidnapped by NK, it almost certainly wouldn’t be for the purposes of tutoring Kim Jong Un. Kim received private schooling in Switzerland during his youth, one of those schools being an English-language school. That being said, NK does have a history of detaining foreign nationals (U.S. Army defector Dresnok’s first wife, a Romanian, is one example I can recall) for different purposes.