"He cried because his mother had forgotten who he was."
Translation:Han gråt fordi moren hans hadde glemt hvem han var.
Not moren sin? Or is that a mistake? I don't want to report it, because I'm not so certain of myself right now...
"fordi moren hans hadde glemt (...)" is a dependent clause, where 'mor' is the subject. 'sin' would refer to the subject, but because 'han' is not the subject of that clause, you'll have to use 'hans'.