Get a small water pistol, everytime he whistles, zap him one. When he says 'Oi, stop shooting me with water' or 'why are you doing that?' turn the question on him with a 'oi, stop whistling' or a 'why are you whistling?' - do it in a fun manner though.
He'll get the message. If that fails, water balloons tend to be interesting.
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Get loads of money, don't need to work anymore, cycle loads
He'll get the message. If that fails, water balloons tend to be interesting.