You're correct, that is not among the delineated responsibilities of the President. However, maybe he's asking this of other Americans as a human being who is saddened and concerned for his other fellow human beings in NOLA.
You'd like to pretend that he's signed an Executive Order requiring all Americans to pray or else.
I am not pretending that Bush has signed an E.O. requiring prayer. I am merely pointing out that his role as President is not to call for religious activity of any kind from the Citizens of the Republic - be it generic prayer, Christian eucharist, Islamic hajj, etc.