Hi Sandy
Whoever that man is he is not 189 Squadron if he was shotdown in 1941 as 189 was disbanded in 1918 and not reformed until 1944. And it never flew Blenheims, it flew Lancasters. It was formed at RAF Bardney, then moved to RAF Fulbeck, and then ended its days back at Bardney in 1945.
The stripes indicate he is a Sergeant (by 1941 this was lowest rank of aircrew, next being Flight Sergeant but I don't see a crown above the stripes). The O with wing indicates Observer which does place him pre-1942, when the role was spilt between Navigator (N and wing) and Air Bomber (B and wing)
K