K.P.
The child needs to be invloved in play therapy with a therapist who specializes in children's anxiety disorders. Selective Mutism (now referred to as Elective Mutism) can be overcome, but it takes time and a lot of patience.
When I was working as a school psychologist, I had three students with this condition. It is incredibly frustrating to work with these children b/c your first "thought" is that they are playing a control game. On some level they are, but it's really about a form of anxiety. There are some children who will refuse to speak as a method of controlling their environment, but again it typically goes back to anxiety.
For children, there are very few things that they can actually "control". In this child's situation, he literally has no "control" over his living situation or what is happening to him and that anxiety must be overwhelming. He is finding control where he can. For some children it is refusing to speak for others it's control over what they eat/eliminate.
Really good therapy and lots of love and support at school- the therapist should have consent to work with the school psychologist b/c you need to have everyone on the same page to make this work!