Depends on when he joined the RAF. Can you give us a date?
There is a minimal MN record for him on the pay to view site FindMyPast. Looks to me he only did one trip aboard the ARNDALE official number 165578. He signed on as an Ordinary Seaman at South Shields on 28/April/1939.
His DOB is given as 15/10/1923 born in Jarrow and his Dis A Number was R186990
As far as I can tell he has no Merchant Navy records after ARNDALE. You may like to see the appropriate Logbook and Crew Agreement of ARNDALE available for viewing at the National Archive.
http://discovery.nationalarchives.gov.uk/details/r/C11026281This will tell you a little more about the voyage and should tell you where he discharged from the vessel .Search via official number.
I may of course be wrong about him only doing one voyage but all the on line MN personal records for WW2 do not mention him.