I think putting him in scenarios where social interaction is required could be beneficial. I had severe social anxiety because of my stutter as a teen , I overcame it by forcing myself into situations where I had to speak to people in my late teens and early 20s - I was a salesmen, a tutor, customer service rep etc... and it really helped me overcome some of the personal struggle of a stutter . Obviously he’s still a kid so a job isn’t the answer but finding ways you can get him to just get used to talking to people and building that mindset of not caring about his stutter will definitely help at that age IMO. I certainly wish I had done more of that when I was younger instead of waiting . It would have really changed things for me in high school when social life really picks up.