It's a standard RAF issue Sidcot Suit (from the inventor, Australian RNAS aviator Flight Sub Lieutenant Sidney Cotton) made from proofed khaki twill over a rubberized muslin inter-lining and a mohair liner. However, he may not have been going to fly in the immediate future, as he's wearing ordinary shoes, rather than an airman's fur-lined 'fug' boots, though sometimes airmen flew without the boots if the flight was to be short and/or at low level.
It seems strange that a man that served in the RFA since 1907 ended his career in the RAF. He remained in the RFA even-though he was sent to the RAF from Jan 1, 1918 to 1919.