I had a similar situation. I was in speech therapy once a week from kindergarten through high school. My speech therapy was only focused on tools to help me get through tough sentences, like easy-on-set for example. I think one of the turning points for me was when I started focusing on the underlying anxiety. Certain things would really testify me like, a teacher calling on me, or getting ready to place an order at a drive thru. And in those moments, rather than trying to calm myself down or be mindful of the situation, I would hyperfixate on the stuttering tools I learned in speech therapy, which only made me less mindful of the situation and stutter more. Everyone stutters for different reasons. But I think you should consider sending your son to therapy (in addition to speech therapy) - with the hope that he can learn more about him self and his triggers. I went to therapy for depression (at the time I thought was somewhat unrelated to my stutter) and was introduced to mindfulness therapy - this has been the number one tool for me day-to-day. For example, when I am waiting in line about to order Starbucks - before I would repeat my order to myself a thousand times focusing on how I would enunciate each syllable to avoid stuttering. Now when I am waiting in line, I read menu items, watch people sitting around, and do not let my mind leave the present moment. I don’t let myself get afraid of future stuttering, and I definitely don’t let myself dwell on past stuttering. It often works for me.