Here is a photograph of the Cox family. The handwritten information on the back of the photograph states that it was taken "circa 1904". If so he would have been 13 or 14 years old however he looks older than that age in this photograph so I suggest the photograph was actually taken in a later year.
He appears at top left in the photograph.
The handwritten caption on the back of the photograph reads Left to Right:
Back: Joe, Grace, Rupert, Alma, Jack, Vi
Centre: Gert, Ede, Grandfather, Grandmother, Darcy, Ethel
Front: Eric, Rene.
Searching the online archive Trove, I found the following pieces of information about the family.
In the 30 June 1916 issue of The Dubbo Liberal and Macquarie Advocate newspaper there is an item about the marriage of John Douglas and Alma Cox where it states,
"Mr. John Douglas, eldest son of Mrs. Douglas, and Miss Alma Cox,fifth daughter of Mr. and Mrs. F. Cox,of Buninyong (were married) ...She was attended by her sister Irene ... The bridegroom was attended by Mr. Joseph Cox, the brother of the bride...."
The 1912 wedding of Mr. John Cecil Cox (possibly Jack? ) to Miss Martha Raines is mentioned in the newspaper here. Miss Ethel Cox gave a present, and the bridegroom was attended by Mr. R. Cox (possibly Rupert?)
The 1912 wedding of Mr. Victor Rupert Bruce to Viola Elizabeth Cox is mentioned in the newspaper here. Bridesmaids were the Misses Alma and Grace Cox, and Mr. J. Cox, brother of the bride was groomsman.
The 1921 wedding of Miss Irene Cox to Mr. J. Morris is mentioned in the newspaper here.