Why must they not be his parents? The fantasy trope of old magical bloodlines emerging from otherwise mundane parents is quite common. You're making several assumptions here to come to that conclusion since 1) we don't know if he's not human or just a human with special abilities, 2) if he is not fully human, we don't know if that's directly hereditary, 3) if it is hereditary, we don't know if it's due to "latent genes" or something or a non-human parent, and lastly 4) if it is due to a non-human parent, we don't know for sure both Roland and Helga are fully human either.