When he awoke, it was due to Rido. He realized what happened, and because of the bloodlust knew that he very probably would drain and kill both mother and father.
[She shakes her head]
Because that was not what he wanted, he used what remained of his flesh and blood and returned to an infant state, while erasing his memories. Mother and father realized what had happened...but they still raised him the same, as their very own.