I am so sorry you do not have answers, however it seems that some of the worst causes are being ruled out.
This is a wild guess on my part, but something I will share because I know a person who experienced this and later regained sight: Hysterical blindness explained here:
What Is Hysterical Blindness? | Enlighten Me
I wish your son all the best in recovering, whatever this is, and a happy life regardless of what comes.