Hi,
This is just like my youngest DD, it started at preschool with her, she only talked through her friend and only pointed or used head movements to questions, anxious and clingy, the staff at the preschool were unsupportive, she was referred to Camms, which after lots of assessments, we were told she was a selective mute.
DD was starting school in the sept 2012, so I made an appointment with the school to discuss her, it was the best thing I did, they have been amazing, so so supportive but they don't make a fuss of my DD, they have a teacher at the school, who comes in 3 times a week, who just sits on the carpet with the class but always next to her, without making it a big deal, just so she has a regular face, then she will encourage her with questions, she won't talk but its to get her confidence up, then they have a session once a week each morning, where she can take something from home and they can talk about it or the rest of group can.
She is doing great regarding her reading and writing because again they don't make a fuss, they are letting her go at her pace, as she is only 4!
Since starting in sept, you can see the difference in her, all because of the schools support, they don't agree with reward charts, or going on about the non talking as it puts pressure on them, which they don't need.
We decided to not continue with camms counseling, as the school has be so great.
I also brought some books on selective mutism which are great as reading them, changed how I do things too.
I think you should go in and chat with his teacher, she should not be keeping him in on breaks, as this will draw attention to him, which is the exact opposite of what he needs.
The best bit for us, is that she spoke to her teacher for the first time this week, only 3 words but when the teacher told me, she was over the moon but just acted all normal with her and asked her to sit down, where as really she said her insides were doing cartwheels!
Sorry if it sounds rushed, in the middle of sorting tea.
Hope you get sorted soon