Complex sentence: While serving in the Coldstream Guards during the last months of Great War, he was hospitalised in Stirling, Scotland where he first became interested in stamp collecting after his father brought his own collection to help his son pass the time.
Split into simpler sentences: While serving in the Coldstream Guards in the last months of Great War, he was hospitalized in Stirling, in Scotland. There he began to be interested in stamp collecting after his father brought his own collection to help his son passed the time.