A few quick thoughts here. Some people stutter more when they’re most relaxed, as they ‘allow themselves’ to stutter when it matters less. Maybe since you know he stutters and he knows you don’t judge him for it, he doesn’t expend any effort to stutter less. I personally would not recommend reading aloud, unless somehow it builds confidence. Finally, the best suggestion I have is to connect him with other kids who stutter. Stuttering is rare, but there’s nothing wrong with him. It’s just like having red hair, or being left-handed.