I have a card that was given to him when he left The Royal Scots. The front of it says, "Army Form D. 402. National Association for the Employment of Reserve Soldiers, &c. Name and Description of a Man recommended for Employment". Unfortunately, this is all the information I have about his service, aside from a portrait of him in his uniform.
I'm interested in knowing at what age he most likely would have enlisted (would 18 have been standard in 1900?) and were the Royal Scots involved in any foreign conflicts during his short time in the service?
The 1st Battalion Royal Scots were involved in the Second Boer War during that period. They were withdrawn from the conflict from 1900 -1903, the same year your great grandfather left. http://www.theroyalscots.co.uk/page/history/
The National Archives could be a good place to learn more about your great grandfather's service. They can be a tremendous resource and great help with learning more about servicemen and their history, especially if you already know his name, unit, birthday, etc. They either have or have access to a large number of muster books throughout the centuries.