In my experience, if a kid loses a parent early in their childhood and the surviving parent remarries, chances of them calling step-parent mom is pretty high. Especially if the step is introduced early enough and is pretty much de facto parental figure.
Late childhood - teens, seem to settle with Aunt/Uncle in referring to their step. I've seen though some kids who used to call them that but shift later on to a variation of mom/dad -- like bio mom is "mama" and step mom is "mom" (but not all do)
Again. Observational basis on my big ass family. So may not apply to all fams.
In Aeson's case, he already views Seph as a mother figure and calls her as such. So narratively I figure that already establishes dynamic and connection, and seems moot to discuss it with him