6th Message = 6th Borough?

Oskar and his father had a long time "obsession" with locating the mythical "6th Borough" of New York City.

His fathers 6th message on "The Worst Day Ever" included the actual moment of his death.

There is an obvious connection here, but I am still trying to piece it together. Any ideas?

Oskar had a theory that "falling man" was his father. He even included it in his scrapbook. Is he trying to make the 6th message disappear, just like the 6th Borough disappeared? He never played that message for his grandfather, even after forcefully playing the first 5 messages, despite his grandfathers plees to stop. He also never shared the messages with his mother (the one thing about this movie I REALLY did not like). The only person he shared the contents of the 6th message with was William Black. Why? What connection did William Black have to the 6th Borough?

After further research into the legend of the 6th Borough, I think it represents a kind of letting go. New York cherished all 6 boroughs, but the sixth was unnesisary. As time went on, it realized it was unnecisary, so it just left. Oskar knows the truth of the 6th message. But his brain needs to let it go in order to heal. He NEEDS to believe that it never existed. He needs his father to be "falling man". If his father is "falling man", than obviously the 6th message never occurred.