This site should be useful to you:
http://www.selectivemutism.co.uk/
(Use the 'Register' link, at the top right, to gain full access to all of the site's content).
Chris
PS: I hope that you find more support than i did, in my first year of teaching, when I had a boy in my class who would not utter a single word in school. It took nearly an entire school year before an educational psychologist would get involved. He then spent an entire week following the teenager, both at school and at home (where, apparently, he conversed normally with his family and neighbours). I read through the many pages of the psychologist's report, which gave an almost minute-by-minute account of what he'd seen and heard, in anticipation of getting some useful suggestions as to how to cope with the problem. Instead the report simply ended with this bland statement: "Conclusion: Stephen is a non-communicator". (I could have told the guy that before he started! - Grrr!!!)