Pilot Officer, Flying Officer, Flight Lieutenant, Squadron Leader, Wing Commander etc are ranks, not occupations or trades.
On their own, they indicate level of command responsibility only, not duties as such.
Aircrew in the RAF were only about 20% of the total serving, and of those, Pilots (vs Navigators, Wireless Ops and/or Air Gunners, and Flight Engineers) some proportion of that. The majority were in groundcrew occuaptions, in Signals, Admin, Engineering, Stores and the like.
Aircrew, including Pilots, were always in the General Duties Branch, but men could be commissioned as officers in the various other non-flying Branches, Admin and Special Duties, eg.
Rank (OR, NCO or Officer) alone cannot tell you what their duties were, without more information.
Suggest if you have name and number, use the London Gazette online to search out at least his promotions.
The level of responsibility of a Flight Lieutenant depended upon the period in the war and the Unit ot which he was posted. Early in the war, a Pilot F/Lt would eg be in command of a Flight in a Squadron of fighters, or even medium bombers.
Later in the war, with larger Squadrons and/or larger aircraft, a Flight commander in eg a Beaufighter Squadron might well be a Squadron Leader, the Squadron led by a Wing Commander - pilots, certainly.
But, depending on date, the Squadron might also have a (non-pilot) Flt Lieutenant in charge of Engingeering/Maintenance, and another non-pilot Flt Lieutenant in charge of the Adjutants Office.