Re the possibilty of his serving in the The Royal Scots Fusiliers. The officer is wearing a grenade badge on his glengarry. If you look carefully you will see there appears to be a scroll at the base of the grenade. Both Royal Engineers and Royal Artillery officers wore a grenade cap badge on certain headdress, with a scroll with the word "Ubique" (everywhere) which is the motto of both the RA and the RE. The RSF grenade badge on the other hand, did not have a scroll at the base, and thus I am pretty sure this officer is either RE or RA, not RSF. Incidentally the fact that he is wearing a glengarry does not mean he was necessarily in a Scottish regiment. In 22 years in the REME my son at various times wore a khaki beret with his REME badge when he was attached to the Irish Guards and a tam o'shanter when he was attached to the Black Watch, and his dark blue REME beret at other times. This officer may well have been attached to a regiment which wore the glengarry, but retained his own headdress and collar badges. Of course if anyone could enlarge those collar badges and see a thistle on them, then we are back to him being in the RSF!
Best I can do.