I am not very good at identifying Scottish regiments (which he is wearing without doubt) but if you go to
www.thegazette.co.uk and type in "John McNicol" and refine the dates to 01/01/1915 to 31/12/1918 there are only a few John McNicols. I think the most likely would be the one mentioned in The London Gazette of 24 Sep 1915, Supplement 29309 Page 9540. "The Highland Light Infantry ...The undermentioned to be Second Lieutenants. Dated 28th September, 1915:- Lance-Corporal John McNicol, from the Lowland Divisional Train, Army Service Corps".
The London Gazette of 5 Feb 1918, Supplement 30514, Page 1802 has, "Awarded the Military Cross ... Page 41.- 2nd Lt (A./Capt.) John McNicoll Sea[forth] High[lander]s". But, aside from his surname being spelt 'll' at the end, I am certain the man in your photos is not wearing the very distinctive cap badge of that Regiment.
There are plenty of other clues such as his tartan trews, his headdress, his collar badges but, as I said, I am not very good with identifying Scottish uniforms.
The final candidate was in the Royal Garrison Artillery so it cannot be him. Hopefully another member can help here.
Good luck with your research.
