I started stuttering when I was 3 due to Pediatric Tramautic Brain Injury. I am now 33 years old and still have it to this day. Unfortunately, there's no guarantee for it to dissipate. Therapy should help lessen it. In rare cases some do go away forever. Best advice allow him to finish his sentences. Be patient and teach him to not shy away from words. Practice don't make perfect but allows progression.