My DS8 is totally like that since preschool. It happened so many times when people are greeting to him and he has no idea who the persons are. He even didn't recognize his summer camp teacher (he spent 1hr session per day with students) after 4 WEEKS. He played with some kids (not in his class) many many times at playground and he still doesn't know their names. Even DD6 knows those kids better than him. I think he is poor on recognizing people and he has no interest to pay attention on social situation.