Stutter is something that won't simply go away. He MIGHT grow out of it, but it could also be life-long. If you want to bring it up with him, then do so carefully. Don't point it out as a problem, or something that's bad that has happened to him. Be gentle, encouraging, and tell him not to feel shy or be scared of talking. The kid is still young to fully comprehend it, but continous encouragment to speak freely should ease him into it.