Take them to Fair Trading (the residential tenancies division, not the strata side).
Failing that, raise an action at the magistrate’s court for a full refund plus costs – that might get the messages across.
Some agents won’t pass on bad news to the landlords unless they have to. A Fair Trading or court summons would pretty much do the trick and if your son goes along an says he didn’t want to take legal action but the agent wouldn’t do anything, he’ll get a lot of sympathy from the bench.