My twopenneth, with teacher's hat on, by the way I was very when I read your post, must try to stay calm.
Firstly, the "there is nothing more you can do at home" is insulting frankly. It's like saying don't bother, as his teacher, I've written him off this year , so as his parents, you should do the same. I'm really shocked by it tbh. There are always activities willing parents can do with their child.
If you can show that your son was doing consistently well prior to entering this class, but has fallen behind whilst in it, then go back in for another meeting and discuss those concerns with her.
I don't know the ins and outs, but I suspect this teacher has undermined his confidence in himself. Therefore it's v important that you carry on boosting his confidence at home.
Sometimes, the problems are not all with the pupil - like every profession, amongst the brilliant and good and good enough teachers, there are mediocre and poor ones.