Does your son talk to other people outside of school (i.e. neighbors, other family members that do not live with you)?
My youngest dd was diagnosed with selective mutism when she was 4. First year of preschool she said two words all year. She played with the other kids but would not talk to them. The administrator even told me that she probably had selective mutism but did not offer me any help. Halfway thru the second year, one of the teachers took me to the admin who finally gave me a card for the city school early childhood intervention specialist.
I had to answer almost 700 questions about dd then they evaluated her. In addition to preschool three days a week, she also went to a communication class one day a week. Also an intervention specialist came to school once a week to see her. This was all free.
Dd was a little chatterbox at home but when we had company she would talk to some but not others. Outside the home, she would hide behind me or just plain refuse to talk. All attempts to make her talk ended with her in tears. I noticed problems as young as 9 months old when I took her to story time at the library.
The intervention from the school really helped. By the end of K she was reading books to the class but she still had people that she would not talk to. Even now at 8, she will on occasion refuse to talk to people. I can see her stiffen up and not talk when someone she doesn't know or rarely sees talks to her. Sometimes she will answer their questions by looking at me and talking to me instead of them.
Contact your local school system and see if they will evaluate your son and get him so help.