@SandyRoe,
To maintain the stance of 'skeptic', I have to insist on evidence, at all times and we can guess that no such evidence can/will be forthcoming, after such an atrocity.
To the best of my understanding, Muslims cannot kill other Muslims, on pain of (death+hell) although ISIS are treating non(trigger-word)s as heretics and circumvent thus restriction.
Logically, if they killed the son, for bring the wrong sect, they would have executed the mother, for the same reason. How then, does the horror story reach the outside world?
Two options then:
i) they put this story out themselves, to scare their opponents
ii) their opponents put the story out to enhance recruitment.