Why is my son’s flight going so far off course?
neither paths shown is either the planed or actual plight path.
The dashed line is shortest distance and the white line is (as it says) estimated based on time elapsed, and previous flights.
To find the planned flight path, you need to look at the way-points
But the short answer, if it is going that far south of the “great circle” (shortest) route, is that they are probably picking up significant tailwind from the jetstream, which would create a fuel savings benefit greater than the cost of flying the additional distance.
And FWIW often we only receive the planned waypoints for part of the flight, so take with a grain of salt - in this case we only got waypoints inside the US, so the whole Pacific part of the route is assumed to be a great circle (which it isn’t).
If you plug that flight plan into Skyvector it plots those know waypoints
which as @obj stated is only the path over the US.
Will there come a time when long-distance oceanic flights are supplemented with Aireon space-based ADS-B data?
Just one plotted position per hour would give a reasonably realistic record of the flight route.