Please remember that there are humans outside of the templars who roam about neverwhere and the boy could very well be MC - the superman story: Fio saved him from a life ending event - either Cronos wanting to eat him or Hera looking to kill him as some illegitimate Zeus offspring - who, as with Hercules or Perseus, will have a very strong and powerful connection to objects of power, precisely swords made of the magic from the lady of the lake.
I am refferencing the story of the guy MC and his babe can rescue in neverwhere from the goons beating him up - some romeo shagging the goddess, some beautyful barrista ”wink” ”wink” shaggging the immortal elf/demon lord, Sauron - with MC the product of such forbidden love.
Fio could have left him to be found like the Moses story, a little kid in a craddle that the templars found, maybe in a temple, maybe on some altar, they would recognize a child of prophecy, and thus the legend of MC was born.
This would explain how Fio lost contact with MC, the child she saved, to ensure all traces of his origin were lost and the denger would dissipate over time. So Fio meeting MC as a grown up, she would not be any wiser ... or she might be, who knows.