Thanks very stormy.
Although my son can get citizenship by descent from his mum, it appears (unless I'm reading it wrong) that if I apply for a dependant child visa 455, which cost approx $1700, then I can apply for my partner visa 309 at no cost, since my wife and I have been married for over two years. Seems too good to be true, and would it disadvantage my son in the long run?
cheers, Matt.