I don't know your child, but I can say that a toxic school situation beats almost any level of new-situation-anxiety-- and honestly, if what your child has reported is true? THAT IS TOXIC; this is a teacher taking what YOU told her, adult-to-adult, and twisting it into punishing HIM verbally in ways that would be unwarranted even if he were being unmotivated in class.
I was gong to write the same thing. And if I am recalling correctly, this is a private school. I can't imagine paying tuition for my child to be treated this way. Not that it'd be okay for this to happen at a public school at all but the fact that I'm paying $$$$$ in tuition would add an insult to injury.
Without going into details, I know someone who developed anxiety around age 6 because of a mean K teacher. 20+ years later, she/he is still on medications for anxiety and OCD. What I'm trying to say is that don't underestimate the damage a teacher can cause to sensitive, gifted children in early years.