I suspect he also knows he mumbles a lot and that people can't understand him, so he speaks more clearly to strangers.
Can you nicely pretend that your ears aren't working when he mumbles, and ask him to "repeat it clearly"?
Our kids both went through a whining phase; we simply couldn't hear them when they whined, and it was a blessedly short phase as a result.
The school psychologist blew me off. He won't tell me the agenda for the meeting: "Talk to your classroom teacher." Grrrrr. Dude, you're the one that called the meeting...
Uh-oh. That is... bizarre. Can you get the school's person in charge of IAT meetings (the EIS or equivalent) on the phone? I don't think they'd call a meeting to berate you for having your kid tested, but failure to answer basic questions seems like trouble to me.
Good luck--
DeeDee
