On his left arm note the absence of a "killick" anchor (denotes a Leading Seaman or equivalent rank depending on Branch [i.e. Gunnery, Seaman, Signals, etc]) and any Good Conduct stripes, one of which is awarded for each 4 years of "undetected crime", to a maximum of 3 stripes (representing 12 years Good Conduct).
Therefore, we can assume your man was:
a. An Ordinary or Able Seaman equivalent rank
b. His length of service, at the time of the photograph, was less than 4 years.
Do you have any indication of his name, Service Number, Branch, etc?
Q98
Fremantle
Western Australia