Hi Dixonclan,
I'm not sure if
Centrelink Home Page will deal with this. I know they deal with benefits so it's worth a look. I know that some benefits can't be received for 2 years once you've arrived.
If the agent is saying not to put it down on the visa, have they suggested what happens when you get to Australia and your son needs the extra care?
I don't know whether you've heard this before but I was watching Oprah the other day (probably an old version) and an actress saw a marked improvement in her autistic son by changing his diet. If you haven't tried it yet it may be worth a try...
Regards,
Karen