Is there an activity in which he is more relaxed and involves peers whose temperaments are more like his? My daughter has a Girl Scout troop in which she is much more talkative and comfortable, along with a theatre group. Both groups are full of sensitive peers with whom she feels more comfortable. They've been a haven for her.
I strongly suspect your stepson is suffering from an anxiety disorder called "Selective Mutism". The best site for information on the internet is "selectivemutism.org" which gives an overview under the FAQ section. There are many downloads which should help the school to better understand this disorder. Also, there are many books which can be purchased to better help this boy (the school should have funds for some of these purchases). In addition there is a forum which you can post/read questions.
What the parents of this child need to do is to contact their family doctor. Ten years of age is "old" for a child to be selectively mute and treatment will take longer because of his entrenched behaviours; however, I assure you, it is never too late. If the family doctor is not familiar with this rare disorder (and many are not), then the parents should request a referral to a specialist with experience in anxiety disorders. Treatment usually consists of intervention, therapy and possibly medication (at his age, I suspect meds will be used). If sm (the term to which selective mutism is often referred) is the issue, this anxiety will not go away nor will your stepson outgrow it; however, he can learn how to manage his fears/anxieties with proper help. I wish you the best ...
this is called selective mutism. i am very surprised that the school does not know what to do to help him. while it is more common in younger children who are starting school, it's not unheard of in older children. it is an anxiety problem, and needs to be addressed delicately and appropriately. the school needs to have an understanding of selective mutism. when i was sending a little girl from my preschool class to kindergarten with selective mutism, we worked out a 504 plan to ensure that special accomodations would be taken for her.
please take a look at http://www.selectivemutism.org/
it is a good starting place for information and resources.
i just found http://www.selectivemutismfoundation.org/- it has a sample of a 504 plan that can be purchased. not your responsibity to buy it, but please bring it to the attention of the school. also, the "about selective mutism" link on the side has some good basic information. good luck in helping your child overcome this. it is more common than you might think. and the good news is that he CAN overcome it. i have seen children who were selective mutes become very social and active children.