He didn't enlist in 1939, he would have only been 14 years old.
Here he is on the nominal roll, he enlisted and was discharged in 1944:
http://www.ww2roll.gov.au/Veteran.aspx?serviceId=A&veter...As Kellie has said, it is unlikely that you would be allowed to purchase his birth certificate - what information were you hoping to find that you don't already know?
Debra