I have never encountered this problem personally but several of my friends had children who stayed silint for no good reason and then suddenly started speaking. Of course he must start speech therapy as already suggested.
I don't know if this will help in addition, but when my kids were small, as they were learning new words, I took their hands and placed one on my lips while i repeated a word, and then on my larynx so they could see how the sound was made.
then I'd place my hand on their mouth, like a game, and feel the sound they were making.
i've just thought: Can he hear properly? Hearing test, maybe?
Also, read to him . Mr Happy is a good one because they learn that quickly and, with repetition, will join in from memory.
Best advice is med advice re: speech, test hearing and repitition after repitition of stories, songs anything. Human contact, not too much T.V. hope this helpsx
since when did your child started the speech therapy? did you find any improvements?
you have to get him in speech but my son is the same age and he does the same thing he cant say anything he brings me a cup if he wants water and he is alone most of the time i had to stop workin so i can find out what is wrong with him so far they didnt tell me much but they know he needs help.
my mom gives time for him. she don't work anymore now because of it. yes they do communicate with him. he listens but just don't make a word.
I don't know if it is autism, but it does sound like he would benefit from a speech therapist. Who watches him while your parents are working? Do they try to communicate with him?