Yes, he is real. I was on my pocket edition once, and was randomly turning my point of view to see different areas, and out of the corner of the screen, I saw Herobrine standing in a cave. When I turned back to the cave, it was gone. So was Herobrine. So, Herobrine is definitely real. I think that Notch tries to tell everyone he's not real either for added surprise when they do see him, or because somehow he wasn't fully erased from when they did have him in the game.