Those people are criminals and you and your son are naive about how they are scamming you.
You are trustworthy people, and these criminals are exploiting that trust by lying to you.
DO NOT communicate anything with these people again.
Tell your son to go to the police and file a police report.
Even though the local police probably cannot do anything, the reason for the police report is so he can show the report to his bank and any employer in the future who might be contacted by these criminals. And these criminals have probably committed fraud against other people too, and are looking for other people to steal from.
Your son will have to call his bank and report the fraudulent activity on his bank card number. The bank will want to see the police report. They might have other customers, now or in the future, who will have this same problem.
As for the legal action, tell him not to worry about it. These criminals will have to expose their real identities to be able to sue him, and no criminal is going to do that. Instead, these criminals will make threats.
And tell him to get another email address, as the scammers will continue to try to scare him with the current email address and he will have to abandon it. He will probably also have to get another telephone number and abandon the old one.
Just around at 255pm today this number has calling me to asking about process of Australia visa and her told me that my outstanding balance is $800, but as advised,and her say able to give me a discount on this amount and work out a solution for me to pay in installments, so may successfully begin mine immigration process to Australia. and her told me that her own company is the Australia Gsm is finance department can any one members can explain to me is this scammer or what ? and her quick asking of on my personal and privacy & credit card information and say that i also can make a easy payment on this balance amount ....