The birth date fits the man staying in the Royal Hotel, Guildford in September 1939 who Carole found. He is one of three Reserve Officers of the Regular Army staying in the hotel which leads me to suspect that they have already been called up and are staying locally ready for embodiment. A check of the London Gazette should reveal more about his commission and his Regiment or Corps.
Interestingly there doesn't appear to be a WW! medal index card for a David Stanley Jones with the number 15650. This could be because, either he never served overseas which seems unlikely, or he was commissioned during the war and received his medals under his officer number. Unfortunately there are too many David S Joneses who fought in the First World War to point to him as an officer. He would not necessarily have been commissioned into the Grenadier Guards (indeed I think it is highly unlikely).