extreme social anxiety

i have a nephew who has extreme social anxiety. He is only 10 years old but is scared of socializing with just about everyone.  He is very intelligent and does very well in school. In fact, he is a consistent honor student.
However, he does not recite in class-refuses to recite even if prompted by the teacher.,  He won't talk or socialize with his classmates. He does not socialize with his cousins either. In family gatherings, he usually sits by himself in an inconspicuous corner. Does not even sit beside his parents. Sometimes his siblings sit beside him to keep him company. But he doesn't interact with them.  He just sits and stares.
What's wrong with him?

[jdtm]

He won't talk or socialize with his classmates

You have described Selective Mutism.  The best site on the internet for information is "selectivemutism.org" - please read the FAQ's first.  By the way, your nephew's anxiety is so great that he is "unable" to speak; he does not refuse to speak.  Feel free to contact this board if you have any further questions.  Best of luck ....