I remember reading this in a collection of short stories. Definitely published pre-2000 (the collection). Not sure when the story itself was written or who wrote it. I remember Isaac Asimov had something to do with the collection of stories itself, but most likely did not write this particular story.
Plot points I remember:
- Early on, man visits alien homeworld, kills an alien and escapes (the leader alien's child I think)
- Man (sickly? old?) has a child (age I cannot remember, likely at least teens?)
- Boy is dating some girl
- There is a species of alien (the letter 'Z' comes to mind) who can mentally project themselves onto Earth to follow the boy. The boy also has some sort of kinesis-like power.
- Man dies while trying to tell the boy something.
- Girl reveals herself to be an alien and that the boy is also an alien but was disguised as a human by the man.