I don't understand your question. You say your son has gotten citizenship by descent, and if his country of birth allows dual citizenship he should now have this. But he'd need to check with his country of birth what they allow.
If you're asking about dual citizenship for yourself, then you'd need to apply for a PR visa if you are eligible for one, and then meet the criteria for citizenship such as the residency requirement of living 4 years in Australia including 1 year on PR visa.