It looks like he's wearing an early RAF eagle shoulder flash 1st worn on the belted khaki tunic uniform issued during mid to late 1918. see
http://en.wikipedia.org/wiki/Royal_Air_Force_uniform scroll down to historic uniforms > initial uniform.
Provided he didn't serve after 1939, you should find his records online, see
http://www.nationalarchives.gov.uk/records/research-guides/r...