User blog comment:FairyRaven/Fairy Tail Chapter 408: The Definitive Demon, Review and Commentary/@comment-7418318-20141110102918/@comment-7418318-20141110140632

Well... with Sai looking more on the androgyneous side, it's no wonder his kid wouldn't be immediately recognizable as being a boy or girl either. But I too think that it's a boy.