A quick sum up of my guessings:
He's clearly wearing mounting boots and a sam browne style belt, so you might have to look trough officers or cavalry units, or maybe rifles.
I tried to identify the cuffs with some unit, as they are damn singular ( or so i think ). Brittish army cuffs at that time (1900-1914) don't usually show a striked or zig-zagged straight pattern without buttons, and the detail hints at either a crown, a rifles knot, or a dragoon knot. I was unable to find a specific unit related to that "color" pattern, but we know that a single line, althought highly decorated, means 2nd liutenant.
The badge on the collar slightly resembles the Rifles (Scottish and Irish) crown badges, so i looked trough and found the 1rst Lanarkshire volunter rifle corps, with some pictures (older) with more detailed knots from the 1850's and 60's, that strike some similarity.
Pieter's answer has one "hit" on the same unit and rank, so i guess we're talking about 2nd Lieutenant Alexander McLennan. I am unsure whether it could also be a young, 18yo enlisted Duncan McLennan back in 1879 in Kabul, given the name match, but it could make sense.
That would set up the next timeline merging what we know:
- 1879: A 18yo Alexander Duncan McLennan enlists in the 92nd
- Later that year, is shot with a slight wound.
- He's probably sent back england to recover, an abdomen wound is not easy to heal.
- 1880: He joins the Royal Military Academy, open to all gentlemen, as cadet.
- After a year of training reaches the rank of 2nd Liutenant.
- 1881: Is assigned to the 1st Lanarkshire VRC on 11th may 1881 as his old unit is merged with the 75th during Childer's Reforms.
- 1881 > 1901: Service with the Cameronians as part of the 1rst Lanarkshire, rank of 2nd liutenant. picture taken.
Do you have any way of asking for the military record on the current unit's headquarters? This unit merged across the XXth century to end up being part of the Current Royal Scots, and they usually keep account of their officers.