'after he leaves' = context is present, but describes just a condition that hasn't yet happened.
e.g. after he leaves, let's set his bed on fire.
'after he has left' = context is present tense, but describes a condition that happened in the (implied recent) past.
e.g. after he has left, he realises that he has left his wallet behind (or he doesn't have his wallet - to make the present tense clearer)
'after he left' = the story context is past. The 'after' has no bearing on the tense change.
e.g. After he left the building, he realised he left his wallet behind. He knew he was a dumbass, but he didn't believe he was this dumb.
The latter part is present tense - so "Long after he has left..." is the right prefix.