dyslexic keyboard:
an a oldie but a goodie. when he's not there, prefably when there's nobody in the office.
- take a letter opener
- lever up a few keys from the keyboard
- put them back in an incorrect way.
with a bit of luck he'll call tech support as somebody at my work once did.
less vicious:
move his stuff around ever so slightly, so that he can't instantly find his glasses or coffee mug.
after he's left work, turn off his monitor and put cap lock on, and watch him try and enter his password in the morning.
don't do it all at once.
there's still more:
adjusting his chair
unplugging his phone cord, so that it falls out when he tries to answer the phone.