My DS1 is 11 and very similar to how you describe your DS. He's always been shy, will rarely speak up in class, or volunteer information unless pressed. He makes friends easily, behaves impeccably in class and is very strong academically, but he seems unable to interact with adults, apart from close family.
Today, for instance, I took something into his school, for his teacher, and he and I gave it to her. She said something like 'Wow, X, that's so kind of you, isn't it great?' and he just stood there. No eye contact, no interaction, or even recognition that she had addressed him directly. I am now focused on not answering for him, so I nudged him and he mumbled something. When I take him to the doctor's, or to an after school club, or into a shop, he seems entirely unable to speak, unless firmly prompted.
He says it's because he doesn't know what to say, and that he feels shy. So we've worked on what generic things he could say to strike up a conversation, or in response, and we talk about what sort of impression it makes when he seems to ignore someone.
It's getting to the point, I think, where he will begin to be overlooked, or signed off as 'ignorant' for not responding and honestly, I am really worried about him starting at high school in September. He's an amazing kid with so much to offer. I don't want him to be sidelined.
Following this with interest for tips and ideas how to help DS!