@msolga,
This may be a good starting point:
http://www.australia.gov.au/Migrants
Also in the "Australia" forum here at A2K there are
many threads asking for information about migrating to Oz. You might want to browse through them & see what you can find. Though, off the top of my head, I can't recall any on dual citizenship. But who knows?
Good luck & feel free to ask questions. There could be others who have been in the same position as your son & may be better able to advise than I can.