As far as I know, there is no such thing as "potential" combatant, you are a combatant or not. The combat pilot, while in the airplane or under the canopy, is a combatant. Though my personal feeling is that the one under the canopy, being generally defenseless, should be (morally) treated as a noncombatant, but I can understand why that's open for debate.
The pilot under the canopy being attacked by a fighter is fully justified to act in self defense.
A combatant once captured, under the Geneva convention, becomes a prisoner of war (non-combatant) and must be careful not to act as a combatant or be subject to no coverage under the Geneva convention.
By the way, the POW that escapes is no longer a POW and not even a "combatant", but IIRC the equivalent of a spy - and until repatriated, subject to being killed on sight by the enemy. It's been a long time since I sat through all these lectures.