No matter when/if he gets his Aus citizenship, his is liable to do NS. By Singapore law, he cannot renounce his Singapore Citizenship before he has complete his NS obligation. Also, if he gains Aus citizenship prior to completing his NS obligation, he is in violation of Singapore Immigration laws as well as being an NS defaulter as it is also not allowed for a Singaporean Citizen to acquire a second citizenship without renouncing the Singapore citizenship (which by law he cannot do until he's complete NS). He will not be able to defer NS beyond A levels, provided they do allow it - which is doubtful. Singapore born minor citizens cannot hold dual citizenship. Only those children born to mixed nationality families with one parent being a SG citizen and the other a foreign citizen. Then they are STILL obligated to do NS but the can hold dual citizenship until they reach the age of majority (21 - they must renounce one of them with 12 months of their 21st birthday).
You will still need to post either a 75K bond or have two local sureties (usually at least one local close relative).
Start reading the following thread where it's linked (actually as this is important, reading the entire thread wouldn't be a bad idea but the link is a current case (this week).....
http://forum.singaporeexpats.com/viewto ... 57#p737957