Hello LeoHB,
Firstly, may I say what a lovely photograph this is and how immaculately turned out he is. See how precisely the plaid is folded across his chest to show the light stripe on each fold. Could the castle on the back-drop have any significance? Edinburgh? Stirling? (Someone will recognise it)
Could the place he was stationed at in Midlothian be Glencorse Barracks, near Penicuik? One time home of The Royal Scots. In 2006 The Royal Scots (the oldest British Regiment) merged with other Scottish Regiments to form The Royal Regiment of Scotland. Glencorse Barracks is now the headquarters of the Royal Highland Fusiliers, 2nd Bn, The Royal Regiment of Scotland. The Museum for The Royal Scots is within Edinburgh Castle & might be helpful in identifying WW1 uniforms.
Also, if your relative was in a Scottish Regiment, his service No. may have started with S, ie, S/1234.
Good Luck with your search.
Regards, Dod.