Open communication is key. If he feels like his stutter is holding him back, reach out to a speech therapist, local groups, anything that he can dive into, and learn how to manage the symptoms of stuttering. I've experienced this and so much more. However, acceptance helped me to move on and stop fighting it. I'm a national trainer traveling the country, facilitating 4 hour PowerPoint presentations training medical professionals every other week in large auditoriums, all with a stutter. My 2 cents, if he doesn't address it on some level, the frustration will grow.