Get a valve core remover from a auto parts store and loosen all the valves on his tires till thier just barely letting air escape. Eventually, the tires will go flat... all of them.
If your really daring, remove the valve cores completely, but put back the plastic caps (this will only work with plastic caps). Loosen the caps to its *just* starts to hiss. Then tighten it a hair so its not letting any air out. With the cap at the point of letting air out, one of two things can happen.
A: When he hits a big pothole, the air pressure will be enough to shoot the cap off and let out all the air real fast.
B: The cap will slowly loosen and the tire will start to go flat. When he goes to fill the tire up and removes the cap, POOF.