My Greatuncle Lieut George Thomson

I have been doing a lot of work on my family tree and have some information from Norton Barracks about his part in the battle of Doiran in 1917. His name was Lieut George Thomson and he won the Military Cross then. I do not understand how he came to join the 11th Worcesters as he was born in the Outer Hebrides and graduated from Glasgow University in 1910. I have his information from the roll of honour at Glasgow University Archives. He was a headmaster in Glasgow most of his life so I do not understand how he came to be in the Worcesters. Can you help? I have put together with the Archivist at Glasgow a portrait of his life combining their information and my family information. He was pleasantly surprised when I moved to live in Worcester!