My advice is to not push him. He speaks, and is not mute, obviously. But is more reserved than other children. He is still very young. If you push, it may make him more uncomfortable, and create more anxiety.
Let him speak at his own pace and when he is comfortable. When he is older (4 or 5) if he still is reticent to speak, you can have a conversation about it with him, and ask him why he chooses not to talk in certain situations. Don't make him feel badly about himself for these choices, but you can inform him that some situations it is considered rude not to speak.. and then you can nicely guide him and help him recognize those societal situations in which a spoken response is appropriate and expected.
But for now, he is still young, and it isn't a big deal. Don't worry or draw unnecessary attention to it or to him.
Give him a moment to respond if someone speaks directly to him, without jumping in to "save" him. LET there be a moment of silence in which he can decide to speak or not... and then if he decides not, you can gently direct attention away from directly on him and be helpful in regards to the person who addressed him.
Do not refer to him as "quiet" "bashful" "shy" etc in his presence. You don't want to create a self-image for him. Let him choose his own.
Good luck.
~Mom to a once extremely reserved kiddo, who is now 13 and quite mature for her age, very well-spoken, and highly regarded by her teachers and other adults who know her. She also is not shy or timid, either. And performs piano in public routinely, with confidence.
Have no fears, Mama.