If you've been in touch with the ADF I'm going to assume you've trawled this site:
which is currently down for maintenance so I can't do a cursory check now. If he served 1950-1960 I think it includes Korea (if he went there).
Do you have a full DOB or only a partial?
If you only have a partial (hence the ADFs advice there are several entries) and you have the names of his parents, how about doing a search for his birth via Trove:
From there, you can get back in touch with the ADF citing a full DOB and place of birth.
Apologies if you've done this already. I'm working on the information you've provided in your original question.
All the best,
p.s. if he has a living sister, assume you have a full DOB??