About all can glean from the photo is that he's wearing the 1908 pattern tunic. This was modified at the end of 1915 & replaced with the economy tunic & this is the one men wore who went in after 1916. So although your man didn't go overseas before 1916 he was a volunteer & not a conscript.
The photo was probably taken in the UK (no bandolier or war saddle) prior to embarkation.
Sorry please could I ask you another question?
My great great uncle who knew William has just told me the only things he knew about William's service:
" The word I had was that due to his
experience with horses, he was put in charge of the stable area where
the horses were used to tow the guns to the front, one of his jobs was
to tow the gun up, do a 180deg to allow the gun to face the enemy lines,
unhook the horse and return to the stable area.
Mum (Ethel) did tell me that one of the things that got him in trouble
was running a crown and anchor game behind the lines and cleaning
everyone out!"
So given what he was doing do you think he might have been in one of the following RE divisions (if they existed as I am just guessing here): field division, artillery division or maybe even the cavalry?
Best wishes Emma