User blog comment:B214/Volume 17 spoilers./@comment-24552694-20140219201202/@comment-9339850-20140220002147

Well The Gremory clan has tried to get Issei to say Nii-san, Oka-san, Otou-san etc. since i think Vol. 4 or 5, cant remember, so it wouldnt be that profound for him to actually say it at this point