Lots of great advice here.
Dove I am so sorry it is so hard.
All I can suggest is this...
Speak to your adoption authority and request post adoption support, if possible especially around attachment, we did this with our son who came to us aged 3 (he is now 6, Year 2).
We also have had Theraplay, this is fabulous.
All of this may address some attachment issues and build confidence.
This work outside the class may help in the class and will hopefully address some of the concerns, it can also be very supportive to you and your son and fun.
Speak ing of you, please get as much support for you (friends or professional as you can, put on your own oxygen mask type thing!)
I totally agree with tldr about finding out specifically what the issues are and then addressing them at home to whatever extent you can.
You are naturally very upset but I must say you are coming across as quite 'aggressive', both in your opening post (towards the teacher) and in your responses to some potentially good advice here.
My advice please feel free to ignore is to please speak to friends, family, and continue to talk to us and get a bit calmer before you approach the teacher.
As a dyslexic formerly shy person I am a school hater and the mum if a dyslexic pupil (our dd not our son) and have no natural love of schools or teachers!
But they do do an increasingly difficult job and they have charge of our most loved so they need to be encouraged to go all out for our beloved kids.
Your best bet IMHO is to go in, ask for a meeting with Teacher and Senco (together if poss) and listen, first, to what they say, make notes.
What does he do, how does he act, how do they think he feels, what does he appear to enjoy, are there some children he does interact more easily with etc?
Express your concerns.
Offer ideas.
Make a plan together or at least together outline some thoughts.
As you would with kids, praise the good. At the moment you can not see any good, but there may be some. So let the teacher and Senco talk first.
Your goal, as you know, is to get the best outcomes for your son, be as gentle as you can while also keeping your feisty inner tiger mama.
Good luck.