My ex husband has submitted an application to move to the US from Australia. He has been waiting for the results for a while now. He has a visitors visa that is valid for 2 years can he wait in the US for the results of his application or does he have to wait in Australia.

NO. Two years visa does not mean he can stay 2 years in US, waiting. For those with visitor's visa (B1/B2), maximum term allowed depends on immigration officers at port of entry, maximum is 6 months.