Once "lethal damage" has been done, damage that comes after it does not count. So if you blow off his entire wing or split his fuselage in half, after that it stops counting. However, if you only blow off the outer wingtip I believe it likely still counts.
Not quite sure where tail shots/pilot kills fit into that.
My personal feeling is if he still has control over his aircraft he hasn't been shot enough and might still pose a threat, but that's just me.
Wiley.