Maseki Gurume: Mamono no Chikara wo Tabeta Ore wa Saikyou! - Ch. 57 - New Stage

[Heavenstar]

Pretty sure that last bit was implied way back when mom explained his origins to him, that he's not technically a son of hers, since they are Dryads.

He is her son. However the way he was conceived was through a dryad's self procreation by removing a piece of herself. A dryad thing.

Basically she is technically still a virgin but Ain is truly her son. Just not through the uhh... familiar way living creatures are born.