I really don't know what you could do, as I been looking for the answer to that question myself for ages without the least idea about how to help DS.
What I noticed though, was that excusing him for the long silences with things like "I'm sorry, he is a bit shy" only reinforced the idea or give him the excuse not to talk (In terms of "I'm shy= I don't need to talk")
I also noticed that he may be playing fine with other children, feeling comfortable with other adults but the moment we say "say bye bye to xy or z" he just clams up. So I guess I should just allow him to do as he pleases as pressuring to say hello/bye bye stresses him out.
One day I explained to him that if he said hello, the other children will get nearer to him as they will know he had something to say and he would have far more fun. I also said that he didn't need to do anything drastic, even half a hand gesture would do, and that I would give him a sticker for each time he did it. He came from school very happy with a sense of achievement, next day was even better but one the third day he just forgot about it
So here we are.... and I also worry about how he would be perceived because of this. Last year teacher even told me that she was worried because he always was in his world, didn't do imaginative play or related to other children and didn't believe me when I said that he did (had to take her the photos of previous nursery but still, I think, she has her doubts! )
A couple of days ago, the head teacher made a fuss at him and then said, "Oh dear, always in a world of his own", if I were a bit more brave I would have said "he is not in a world of his own, he is just blatantly ignoring you!" obviously if instead of saying Hello my little darling how are you doing today?, she had said, have you seen the rabbit I have in my hands? he would have looked up in an instant!