When I taught writing, I called it "knowing your audience." You write/speak differently to your professor than you do to your best friend than you would to a small child.
With that said, though, you should be yourself regardless of whether you're speaking to your professor, to your best friend, or to a small child. It's not about who you are, it's about how you present who you are and how much of yourself that you present. Those are very different things!
Talk of "holding himself back" has an insinuation that I don't like at all. It would have bothered me, too. But as he gets older, I don't think it's a bad idea to begin to talk about getting along with others, with the understanding that kids his age are not necessarily his true peers and he doesn't have to be buddies with all/any of them, provided he has some buddies somewhere regardless of age.
So I guess what I'm saying is that I agree that what your father's wife said was annoying and wrong-headed, but maybe you can use it for good instead of evil?
