Hi Gail! Let him find his own way. These things have a way of working themselves out. Unfortunately, all over the world, kids are taught to fit in, or play with other children or speak up more in class. It’s never the quiet ones that are focused on. They have a POV too.
For the last 6–7 years my family has seen me as this strong-headed, outspoken (often bordering on offensive) character but I know that I have suffered from a lack of confidence for the most part of my life. I only hope more introverts don’t suffer from it. Being the quiet one is often perceived as an impediment more than a quality. I know that my parents have encouraged it in the past. Let your son know that it’s okay to be the quiet one too, but yes, nudge him to interact from time to time.
Never let his lack of interaction become an impediment for him; he’ll find his own way, I’m quite sure.