Periodic awards often did not come with citations, at least in the Second World War. Not sure if the practice was different in WW I. There were a LOT of medals distributed - the Tenth Battalion won 60 MMs for the Battle of Hill 70 alone. Awards were much more common in the First World War than in the Second. Given that many Second World War awards were not distributed with citations, I find it possible that it was the same for First World War awards.
When the Military Medal was first instituted, a full citation appeared in the London Gazette, but when the lists began to reach 100 or more, the Gazette just carried an entry that "The King is pleased to award..." with the long list of names attached.
In some cases (in my experience very few) you might find a citation in the personnel file of the individual. But in almost all cases, the file just carries the issue number and date of the London Gazette.
You might find his name on the London Gazette on-line data base:-
I have a book which lists the Canadian MM winners in WWI. Citations are harder to find. You can also view the War Diaries on-line and if you are really, really lucky, you might find something out. Give me his name and regimental number or unit, and I will help you out.