please do the following checklist before concluding on the child's state or condition.
1. where is developmental milestones age appropriate for speech.
2. when he first started speaking is there a slur,lisp or stammer
3.how was he in communication until you noticed this.
4. try to figure out events or incidents which may have caused any trauma for the child hence the silence
5. any sudden incident that has caused fear
6. has he got friends
7.how is he with other children
observe him for a week or a fortnight while you also nurture his love and confidence before taking him for all the clinical testings. gud luck

I used to care for a little girl that done this.When she first came to my Day care, she wouldn't speak to any adults only her peers but not often. I told her mother about selective mutism. The children choose not to talk. Instead they whine or grunt. in her case she came from another daycare where her hair got pulled out supposedly by a child. Which got reported. In her case I told mom that She act as if she was afraid of adults. Therefor we have to gain her trust. Every day I told her that I loved, that I would never hurt her and if she ever needed me She could come to me. Granted this child was almost 4 still not potty trained the other provider said that she pottied for her. The child wouldn't even leave the table after eating unless I ok'd it. I told her that she could get up any time she wanted. I have her in my care for three years and you could not shut her up.

I'm not saying that this is what your son is going through. but this little girl had all the signs of fear.

I hope my story helps.

This could be selective mutism. That's an anxiety condition. When he does talk, how is his speech? This condition could cause some delays his language development. You should probably ask the school district, or a private speech pathologist to evaluate him for speech disorders. And you might also want to consult a developmental pediatrician, ora child psychiatrist to evaluate him for anxiety.

Here's a discussion board for speech disorders:

messageboards.ivillage.com/n/mb/listsf.asp?webtag=iv-ppspeechdel&nav=start

Here's a site for selective mutism:

www.selectivemutism.org/

another:

www.selectivemutismfoundation.org/

If you google "selective mutism" you will find many resources.