I worked with a little boy at preschool who was diagnosed with selective mutism aged 4.
Like your DS he was a very chatty and inquisitive little boy outside even if he saw his teachers out and about but in the classroom he was completely silent.
Our boy was referred to SALT who worked with him to develop techniques that he could use to keep calm and spoke about ways he could communicate non-verbally his progress reports and a brief outline of each session were passed to our manager in order for us to continue these techniques. We had a little flipbook containing pictures of things he may need to ask for (toilet, handwash, cup, toys etc) that he had access to at all times and we made a conscious effort to include him at circle time by asking group questions and asking him directly giving him a brief period to answer before moving on and thanking him for taking the time to listen.
As time went on he would say a few words every now and again whilst we were playing with him, it was like he relaxed and forgot where he was for a minute or 2, we didn't make a fuss but instead just chatted back and forth with him until he stopped.
The little boy is in primary 2 now and still takes a lot of coaxing to speak in class but he's much better than he was and will put his hand up to answer questions or call on the teacher sometimes :)
Your DS' preschool don't sound very supportive in this so I think you need to speak to them about this, they shouldn't be getting angry or criticizing a little boy who has anxieties around them and there is no reason they can't complete his achievements non-verbally unless they are specifically talking excercises.