Really? I've only witnessed children calling their step-parent by their name. They'll only refer to their step-parent as step-mother or step-father when introducing them to people. I think it seems more plausible if the child were 4 though not 8.
That's fine, but If the MC wants to know the depth and impact she had on him to the point he wanted to call her his mother, it's a valid question when he's already told us he calls Persephone his mother. It's not moot to want to know as to what she's done to affect him so strongly. We're trying to get to know and understand him better. It's not like I or my MC would say he's need to stop calling her that, that would be disrespectful.
I mean, in the grand scheme of everything going on, yeah it is. I just personally don't understand it cause from my experience it's a rare occurrence. So I want to know the depth of why he does so, but it's not that serious.