Akira was pretty crafty, to come up with such a lie to make Touma willingly wear it. Unsurprisingly Natsuki, the 100% straight man, couldn't get the hint.
I wonder if it is just an ordinary stomach band or whether it's some sort of questionable health/fitness device that one of the Minami brothers might have bought off of QVC (a home shopping channel which does have a Japanese version, QVC even had the naming rights to the Chiba Lotte Marines baseball stadium for a while)?