Identify him as "miss" in all official forms and data entries. Whenever you are buying a airline ticket, making a reservation, or filling out a form in any place, be sure to make him identify himself as "Miss".
This is just to tease him in a small way, as this information will come up again when he least expects it. He will be addressed as "Miss" on mailing lists when he receives things, and he'll always wonder what the person behind the check in desk must be thinking when they are looking at his reservation. in some cases he might have to even explain the error. This is just a way you can tease him using public humiliation in a very discreet and fun way, even if you are not around.