If the demon was their son, it means they changed the initial depiction of the demon the Mage and Guru killed; from a tall, slinder man with long dark (maybe black) hair to a man with Shin's build with shorter hair. Are they implying that Shin's father was the demon. And if all of this is the case, wouldn't that mean his grandfather was lying in the first chapter when he said they weren't his real grandparents and they don't know who his parents were?
Edit: just re-read chapter 53. They changed the demon again there. In that chapter he has slightly more than neck length light (maybe blonde) hair. Come on! Is it too much to ask for consistency? They also changed young Merlin Wolford's design. In chapter 2 he had light hair, but in later depictions he's shown with dark hair.
You're walking on thin ice, author.