I don't think he messed up at all. He made an assumption, but you had left the gender open so it is pretty natural to assign a gender in your head. I don't really understand why you kept it a gender mystery. That is important information that you probably need to share with him. Why not consider telling him basically what you wrote about your decision to not give the gender and you reacted when he mistakenly used the wrong gender (again, an honest mistake).