Can I just say also that you will NOT be allowed to leave the country without paying the fine! This happened to a fellow Australian friend - her two children's visa's were not extended and were overdue and when they all tried to go back to Australia, Immigration at the airport departure would not allow them to leave. It was a mad midnight dash to the other Immigration office to pay the fine - 10,000QR!!! Sorry to tell you this but forewarned is forearmed.
I really wish good luck for you and your son - it is terrible predicament that you find yourselves in. :(