Selective mutism, formally known as elective mutism, is a disorder of childhood that is characterized by the peristent lack of speech in at least one social situation, despite the ability to speak in other situations.. Onset of selective mutism typically occurs before a child is 5-years-old. However, it is usually first noticed when the child enters school. Specific features of this disorder are described in the 2000 Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR) (pp.125-127) as follows:
Consistent failure to speak in specific social situations (in which there is an expectation for speaking, e.g., at school) despite speaking in other siturations.
The disturbance interferes with educational or occupational achievement or with social communication.
The duration of the disturbance is at least 1 month (not limited to the first month of school).
The failure to speak is not due to a lack of knowledge of, or comfort with, the spoken language required in the social situation.
The disturbance is not better accounted for by a Communication Disorder (e.g., Stuttering) and does not occur exlusively during the course of a Pervasive Developmental Disorder, Schizophrenia, or othe Psychotic Disorder.

Associated Features and Disorders

A number of different psychological and personality features have been associated with selective mutism (e.g., excessive shyness, fear of social embarrassment, social isolation and withdrawal). Different studies identify different features. Even when a group of children with selective mutism in a particular study shows a tendency toward a particular feature, there are still children in the group who do not display that behavior. It is the persistent failure to speak in particular social situations that is the hallmark of selective mutism. There are different degrees of severity for both verbal and nonverbal communication. The severity also may vary from setting to setting. Children with selective mutism usually do not have speech or language problems; however, an associated communication disorder (e.g., articulation or phonological disorder, receptive or expressive language disorder) may occur. These communication problems though are not the main reason for the mutism.





There are a host of other issues that go along with this disorder. I am glad that you chose to be mature and to learn about it rather than ignore this child, or his affliction.