Sounds very similar to me when I was around 12-14 I told my mum too that therapy wasn’t helping and till this day I think my stutter has just become better overtime naturally and MAYBE because of my therapy sessions but I don’t ever use the techniques in my day to day life that I was taught. However if your son is genuinely not affected in his day to life and social life I say let him be and if he ever wants to go into therapy then let him do what he wishes. I hated speech and language therapy as it seemed to not be helping me at all and I was dread going, really made me upset and miserable so please don’t force therapy on him (I’m sure your not but I’m guessing you understand what I’m trying to say).