I feel that this soldier is London Scottish, sometimes when regiments suffered large casualties in battle remnants of the regiment were transferred to other regiments, perhaps that is so here, but an Argyll and Sutherland Highlander wearing the black watch kilt on very old ww1 pics the tartan is still visible, I can't see any tartan in your man's kilt and that suggests London Scottish who wore a plain Hodden Grey kilt and the plain black Glengarry.
It took me a while to reply as I wanted to be sure.
James
