Translation:He could not fathom what had happened.
The English word "fathom" is a little mystic for me. Is it really used in this context?
in English we often use "fathom" to mean "work out" or "understand" or "comprehend", as in "he could not work out/understand/comprehend what had happened". Confusingly a fathom is also a unit of depth equal to 6 feet, although its rarely used in day to day speech. Unless your on a boat
The original meaning of fathom (Old English fæþm) was an embrace with the distance of one's arms. It eventually took on the meaning of mental grasp or understanding, or the measurement more specifically. It is cognate with Norwegian favn.
Does "som" add any meaning here, or does it just fill the place of subject in the object phrase? (and if so, is it necessary?)
I think it's just omitted in the English sentence. You could say "... that which had happened" and "... what had happened".
How do you say the following in Norwegian: He could not have understood what had happened.
I'm guessing the sentence would be: Han kunne ikke ha forstod hva som hadde skjedd.