This is the most awesome prank if you can setup before they come to work. Also does he have an LCD monitor? If so let me explain what you want to do.
For this prank you need,
metal coat hanger, small cardboard tray, tape, a mouse trap, fishing line, glitter
1) Cut the metal coat hanger, and bend it into shape.
Depending on the make and model of the monitor, the length of the wire will need to be cut accordingly. Once the wire is cut, bend it into shape and tape it onto the spring- loaded bar [Figure 1].
2) Use cardboard to construct the launching platform for the glitter.
Once assembled, tape the box onto the spring-loaded bar [Figure 2].
3) Fasten the mousetrap to the back of the monitor and then tie the fishing lines in place.
Try to keep it out of view, this could be tricky and may not work in all offices. Before setting the trap, tie one end of the fishing line onto the mousetrap lever and then fix the other end onto the mouse cable. It is important to move the mouse out of place, so your victim will reposition it once he or she sits down. Then set the mousetrap [Figure 3].
4) Delicately pour a few tablespoons of glitter onto your cardboard launch pad.
Try not to set off the trap when pouring the glitter. If all goes well, the trap should spring once your mark moves the mouse back to its original location.
http://i267.photobucket.com/albums/ii302/Audiparty/Austin_cubicle_monitor-trapgif.gif