I really don't understand Denise's post about not believing someone posted about selective mutism. It IS a possibility.
At home my dd would talk nonstop but out in public, she did not talk to us if there were others around even some other family members.
My youngest dd (now 7), did not speak with anyone in her 3 year old preschool class and only said two words to the teachers the entire year. The administrator even mentioned to me about the possibility of her having selective mutism but never offered me any assistance with it.
When she was in the 4 year old class, one of the teachers pulled me aside after school was in session for a couple months. She said that something had to be done because my dd was not talking to other kids or to the teachers. She took me to the administrator who pulled out a business card for the city school's early childhood intervention specialist. I really don't know why she didn't offer me that help the previous year.
I had to answer approx. 700 questions and have my dd evaluated. They diagnosed her with selective mutism and an IEP was set up. My dd participated in a communication class through the city schools for 2 1/2 hours once a week for about 3 months. Dd did start to talk to her teachers and her friends. After preschool we would sometimes have playdates with her friends and I had told her that we would not go if she didn't talk to the other kids. She did slowly start talking to them after that. But if she saw an adult look at her, she would stop talking.
By the time my dd was in Kindergarten, she had gotten over not talking to other students and to the teachers. However, she still would not talk to some family members. Her Kindergarten teacher actually did not know about the IEP until I had told her about it during open house (about two weeks after school started). The teacher and I decided that we would keep the IEP in place but not follow it as far as pulling her out of class once a week. We would only follow it if she went back to not talking. We ended up not needing to follow the IEP. By the end of the school year my dd was reading books to the entire class.
At the age of 7, she is still somewhat shy around people that she doesn't know especially if they talk directly to her and ask her a question. But she can also be quite talkative and sometimes silly with other people around as long as they aren't talking directly to her.
Check with your local school system and see if they have any programs in place for early childhood intervention. They should be able to direct you on how to handle your son's shyness.