They (the DPRK) are under strict obligation to notify international civil and maritime authorities of their intended launches, trajectories, launch times and touch down times, elevation, speed, etc, etc. They are under complete abrogation or violation of those basic international agreements. I do think that if your nephew's air route was LAX to Incheon Airport (S. Korea), then the (Sea of Japan) transit for his A/L was much lower latitude-wise, it would seem that he would come into Korea across a lower transverse mid-section of Japan, crossing something like Kyoto or Fukui, or Niigata, or Ishikawa/Kanazawa on the west coast of Japan, over to the central east coast of South Korea (then across S. Korea into Incheon), well away from the trajectory, well south of it.
It is the US flights from Chicago or East Coast areas to S. Korea which will take the aircraft(s) much further north (northern Alaska), then down the Sea of Japan, transversing it, and running into the potential touch down areas for these missiles and their 10 minute flights.Still not very settling, is it.