just loosen the sump plug to 1 turn and give it a week...the vibrations will shake it loose!........but prawns in the air vents is a good one.
or even just let his tires down ...all 4!
grease under the door handles is good!
tire a plastic card to the back of one of his mags, and as the wheels spinns the plastic card flicks the strut (makes the noise when tiring a baseball card to the spokes on your pushbike).....will having him think his wheel will fall off or have a flat!