You can sign up on Trainline etc for date of early release of Christmas/New Year tickets. Their website should tell you. Of course, that could be cancelled this year as travel more restricted. If they are doing it, your son will need to be on the site the second it opens. They go fast, especially the cheap ones. Tell your son to have a look at going back via the Peterborough/Stansted line. It runs into Liverpool Street or Tottenham Hale and tube from there. A bit roundabout, but if it gets them home. My daughter is driving up simply because of train works into Kings Cross.