Responses
So when he's around friends/family, he doesn't feel pressured to 'speak properly', so he doesn't stutter.
When he's with strangers, he may feel he's going to fuck up and stutter - therefore he gets nervous and it almost becomes a self-fulfilling prophecy. The more often it happens and he stutters when he speaks, the less confidence he has in himself to speak 'normally' in a setting with strangers. :)

I'm slightly confused (as I don't know much about stuttering). It says that he's not stuttering out of nervousness, but he's nervous because he stutters. Why then doesn't he stutter around family or by himself?