dropping off the monster

This morning I dropped my son off at school and every day one of his three teachers meets us at the door they always greet him "Hi monster" and he never answers back till I say to him Monster say hi. To which he replies Hi or good morning I always thought this was a morning thing and that he only did it while I was a round until I had this conversation

Monster and I get out of the car and walk in to the school we are met by teacher # 1

T1 "good morning Monster"

................................

ME: "Monster please say good morning"

Monster "good morning" with a empty look on his face

I then look at the teacher and ask her if he ever says good morning or if he always has to be prompted to respond at which point she tells me he always has to be prompted even by my mother in law after he is picked up from school to tell them good bye??? I then tell his teacher I'm sorry then tell monster that it is rude not to respond to someone who is speaking to you and that he should acknowledge someone when they are talking to him.

I don't know why but this really bothers me. Like I don't have the since or smarts or manners to teach my son not to be polite. Which he is.... he says thank you and ladies first and your welcome although he does still talk with his mouth open (not from my lack of nagging). So I don't know I'm going to talk to him more to night and figure out what is up with the rudeness!!