I'm not sure what you mean by 'circumstances beyond your control' but as far as I know once he turns 16 he doesn't legally have to stay there if he doesn't like it. And practically, there's not a great deal that can be done about a teenager (even less than 16) who wants to live with one parent rather than another.
As Welshtony and others have said - and without hearing your agent's side of the story - this sounds like really bad advice.
Do you realise that if you sponsor for a child visa, the *first* thing DIMIA will look as is whether your son was mentioned on the original migration application, and if not, then why not.
Is the agent you used MARA registered?
Your son should have done medicals at the time (even if not included in the visa application) and then you could easily have sponsored for a child visa while he's under 18. Parental consent would not be needed if you could get a solicitor to demonstrate it's not required under UK law (as far as I know, those aged 16 and 17 in the UK are fairly free to make up their own minds).
Now you've got a problem. However I would concur with Welshtony and Gill that with good professional advice it may be possible to ensure a good outcome. Almost certainly you will need to "come clean" with DIMIA, and you must get legal advice before that.
Gill has mentioned a reputable pracbreastioner in Sydney who could help, another lawyer I'd recommend for a case like this is Peter Bollard (also in Sydney):
Your validation deadline is another issue you have to negotiate. It's actually a condition on the visa rather than a legal expiry date, DIMIA normally cancel a visa if not validated before then, however in special circumstances they may decide not to cancel. Someone like Peter Bollard would be able negotiate in this respect.
The bottom line is that you need some urgent professional advice to sort things out. There is not a day to waste.
Jeremy
-- This is not intended to be legal or professional advice in any jurisdiction
