I think the cap he was wearing was a Balmoral,
because i have just been reading A G Wauchopes, vol 1, p201, it says :-

( For the 2nd battalion regular BW :- )

A type of 'bonnet' or 'Tam o'shanter' was introduced in the winter of 1914.
This was of blue cloth and very small in circumference: a khaki cover was worn with it.
By the month of May 1915, the dress was :-

Balmoral bonnet ( blue, with khaki cover and red heckle )
Jacket, field service, khaki serge
Kilt, ..................................