Does my son need a visa if I am travelling to Australia, he is a US citizen, we have a one way ticket?

I would say yes. The Visa would get him into the country, then from there, he can go abouts becoming a citizen.

Your son definitely needs a visa. You can get an Electronic Travel Authority (ETA) for free. Maximum stay on that is 3 months though. You can get it at: http://www.eta.immi.gov.au/

Or, if you are going to be there for longer than 3 months, he'll need to get a Tourist Visa (subclass 676).
Check out the Embassy of Australia (for the USA) website at: http://www.austemb.org/whwh/home.html
There is a wealth of information here :)