Bar Mitzvah Bewilderment

The following story takes place many years ago in Europe. It’s a true story that just hasn’t happened yet.
Shimon: My father told me that my bar mitzvah is this week Parshas Noach.
Boruch: Mazel tov! I can’t wait for the yummy kichel after you lain.
Shimon: I don’t think I’ll be laining my parsha.
Boruch: Why not?
Shimon: My father went on a business trip and never told the gabbai when my bar mitzvah is.
Boruch: That’s no problem. Just tell the gabbai yourself.
Shimon: The gabbai says he holds like the Siddur Bais Oved, that a child isn’t believed to say that he himself is bar mitzvah. Only a child who we know is already 13 is believed for that. Obviously, if we know he is 13, then there is no need to believe him!
Boruch: The Kaf HaChaim holds that a child is believed to say that he himself is 13. Further, I had a dream that a great gadol named Rav Shlomo Zalman, zt”l, will be born, and he will agree with the Kaf HaChaim.