
"British Subject" born in Wales, or born in Queensland .... no difference from an enlistment point of view, regardless of his parents divorcing at some time AFTER his birth, presuming his parents were married to each other at the time of his birth. No such thing as "Australian Citizenship" until 1949. (Same for the other dominions eg Canada, NZ, South Africa ... )

There were a number of British Dominions that provided death certificates for the Fallen. Queensland, New Zealand, and some others. Many of the Queenslanders are indexed for the year 1922, including several who were Fromelles missing, and researched on the Armed Forces board here at Rchat.
Do you have a copy of the English GRO cert? I suspect it will be scant on info, but just perhaps it may give a place of BIRTH as well as the usual address of his parents .... Fingers crossed, the GRO certs can be expensive and give little info.
Cheers, JM