I’ve been stuttering since 2nd grade and kids are mean but it made me strong so I’m sure he will turn out fine with the right therapist, the right support from his parents and some good affirmations. Try making him read out loud and tell him to slow down when he speaks- remember to breathe!! His stutter is nothing he should feel bad about and the “friends” that make fun of him are NOT HIS FRIENDS!!!! He will find people who don’t laugh at him and those are his people.