Parents treat stuttering like some boogyman where if they don't acknowledge it it doesn't exist. The main message for your nephew, and even more so for his parents is that stuttering doesn't have to be seen as something bad or a disability or whatever. It's a difference. A different way of talking. Stuttering without the baggage is just disfluency. The key thing is that your nephew doesn't learn and internalize negative feedback from it, especially from home. It's just a different way of talking with some more bumps or however his disfluency shows up.