Whichever hand you used to cast the spell.
If it's the primary hand, have the PC do everything at the table with only his off-hand, to represent the difficulty of not touching anything and thus discharging the spell. Rolling dice, looking things up in books, procuring snacks - all must be done one-handed to represent the dfifficuly his character is having with similar feats in-game.
If he forgets and uses his primary hand...zappo, spell is lost. If he sets his hand down on the table to rest his arm, spell is lost. If he bumps another player, or scratches his nose, somebody is taking damage.
That sort of object lesson ought to drive home to him exactly how silly he's being.
Oh yeah, definitely don't forget that if he casts another spell, the shocking grasp is lost.
J