Yes he can enter using his Australian passport. Are you compliant with your RO given that your child is 3? If not you can't sponsor your child until you are compliant. In most provinces your child will not qualify for provincial healthcare. You also may end up having problems with school registration if your child is not a PR. It will depend on the school district.
Thank you very much for reply. Yes we are not in compliant with RO. Our card expiring July 2021 and we have lived only 10 days since we got our PR.
I have a query if you could answer, please
1) In this case if we enter Canada ,my son visit visa would expire in 6 months. Can i extend it in Canada without being exiting since we cannot return back after Jul 2021.
2) Can we get private health insurance for my son and if yes. Do you know how much it cost roughly?
3) My son is 3 so will see schooling after a year and half . I see some school do allow if your residency status is unclear. Not sure if thats gonna work