I just talked to my son's music teacher!
Since playing tenor sax is her idea, she's willing to provide the instrument. She wants me to provide reeds. I'm cool with that!
As far as his progress in class... she says he has "occasional flashes of brilliance and the rest of the time he's in the middle of the pack".
She wishes she could see some enthusiasm from him; just a "moment of real happiness" would be nice. She says my son is "a problem", but then clarified that she meant "if there's a potential problem, your son is always the first to spot it, no matter what it is!"
So... while I don't get the sense that she doesn't like my son, I can see how he came to that conclusion. He probably does irritate her!

I told her that my son's moments of joy are low key and subtle. You have to know what to look for. Also, I pointed out that he has diagnosed anxiety issues, so what she may be reading as him being negative is just the boy having an attack of nerves.
However, on the plus side, I told her that music is his favorite class. Also, that he never has to be reminded to practice. I take THAT as a much more accurate sign of his engagement than whether or not the kid ever cracks a smile. He does love music, even if he doesn't always show it.
(His violin teacher has commented to me, "He does not seem enthusiastic at first, but when he plays he is all focus, all business. You can see the love there." She's Ukrainian, though, so maybe she gets his personality better.

)
So it looks like my boy will be playing Tenor sax!

And I'm going to have to increase his practice time. I'm looking forward to that, actually. I love hearing him play.