21 year old son

I came to australia, ACT over 2 years ago with my son and daughter on a 457 visa.  We were originally only given 2 years so applied again when the two years were up and got 4 years (bizarre) I thought a 457 visa was only 4 years in total.  Any how they are not letting my son go on the 457 again as he turns 21 in March this year, but i think its wrong as he came here with me two years ago and now they expect him to go back to Scotland where he has nothing and gave up his life back home there to be with me and his sister.  He has struggled to get an apprenticeship due to his visa being short term so has only been working intermittently and soley relies on me for money, food and a roof over his head.  Does anyone have any suggestions as his visa was only extended till 14th March 2014 as he turns 21 on 15th March, im at my wits end here as cannot bare for my son to go back home to nothing.  Any help assistance would be much appreciated.