Nobody knows the reason for stuttering. There have been lots of theories that suggest that stuttering is genetic, neurological, or psychological. Unfortunately for us stutterers, that means that there isn't a "cure" that would fix our stutter. Nonetheless, many stutterers have found ways to manage or control their speech, in such that it no longer has control over them. From personal experience, I have found speech therapy to be useless in the long term. I have been to therapists in my childhood and teenage years, and during both those times, I would be "fluent" in the therapist's office, but fail to use what I have learned there in real life. Therapists approach stuttering as a dysfunctional speech mechanism and try to fix it by teaching their clients how to speak from the start. The problem with this approach is that stutterers know how to speak, their problem isn't a speech dysfunction, it's more complicated than that. Ask your son if he stutters when he is alone, when he sings, when he speaks to children or pets, when he's with people he is most comfortable with. The only thing that has worked with me is [The Mcguire Program](https://www.mcguireprogramme.com/en). It's a program that was started by a stutterer, and now has thousands of stutters who help each other overcome it, and take control over their speech. The transformation of people on the program is nothing short of extraordinary. I recommend you (and your son) to read more about it, watch videos of members, and contact them if you have questions. Of course, it is not for everyone, it requires commitment and discipline that many people find overwhelming. One more thing that might help you better understand the question of "why does my son stutter?" is an [speech/article](http://www.masteringstuttering.com/recovery-stuttering.htm) written by John Harrison. He also has a book called Redefining Stuttering, in which he explains this concept in more detail. His approach to stuttering is also very similar to the McGuire program's and they are covered in their courses.