Does he talk freely and openly to people? Does he answer people's questions when asked? There is a what is called Selected Mutism. It is absolute, painful shyness.
If the above questions pertain to your son, check out:
http://www.beachpsych.com/pages/cc76.html
Here is a selection from this webpage:
Selective Mutism
Selective Mutism is a psychiatric disorder most commonly found in children, characterized by a persistent failure to speak in select settings which continues for more than 1 month. These children understand spoken language and have the ability to speak. In typical cases, they speak to their parents and a few selected others. Sometimes, they do not speak to certain individuals in the home. Most do not speak in school, and in other major social situations. Generally, most function normally in other ways, although some may have additional disabilities. Most learn age-appropriate skills and academics. Currently, SM is thought to be related to severe anxiety and social phobia. SM may be associated to a variety of things, but the exact cause is yet unknown.
These children may respond, or make their needs known, by nodding their heads, pointing, or by remaining expressionless or motionless until someone correctly guesses what they want. The majority of these children express a great desire to speak in all settings, but are unable to do so due to anxiety, fear, shyness and embarrassment. Many do participate in activities non-verbally. The withdrawn behavior is not usually disturbing until the child begins school. Sometimes, even then, the child is viewed as shy and is assumed that the shyness is temporary and will be outgrown. By the time SM is recognized (if it is recognized), the child has usually had at least 2 years in which non-verbalization has become a pattern. The behavior then becomes increasingly difficult to change because the child has found a way to avoid the anxiety of speaking.
The first symptoms of SM usually begins between the ages of 1 to 3 years. These symptoms may include shyness, a reluctance to speak in some settings, and a fear of people. Although onset is usually before age 5, the symptoms become noticeable when the child is requested to respond verbally in social situations, including preschool, elementary school and community environments.
SOURCE: Selective Mutism Foundation