All,
Interesting pic. Although they all 'look' Military, the only one standing with his feet in the proper Military stance for the era, if I recall it correctly, is the one man in question. Look at his left foot, it is correctly indexed with the heel pressed into the right foots arch and his rifle is at the correct 'rest' position. His 'Metis' coat and floppy hat may indicate a 'prize' taken on the Field of Battle, but as the others above have already suggested, he does look a tad bit different than what has been suggested as 'the Man'.
If it wasn't for the 'Metis' styled coat, it could almost pass for a group of Commandos of the first Boer War in Southern Africa. The Boers wore large floppy hats similar to that and canted in the same fashion, they also wore full beards and dressed much the same. The pic could easily be that of an African Boer working as a Scout for the British if R.J.R. did not have substantive proof otherwise, and this is one way how old photos are often misinterpreted.
Now with that in mind, one is wont to say that under the harsh circumstances they experienced during that timeline, anyone's visage is subject to alteration and change due to the weather, injuries, campaigns, etc., so, it may indeed be him but at a much later time.
Just my humble observation after perusing the pics, right or wrong it does not make any difference, it is still an outstanding historical photo and worthy of discussion on this most enjoyable forum.
Best regards and good researching!
'Ol Gabe