This is a tough one. Maybe just encourage him to speak slower and softer and don’t talk about stuttering with him, The more you build a mental framework around it the more it will get imbedded & habituated and the more difficult to let go of. Also look into a low inflammatory diet. Inflammation causes tension in the body and the more tension the more someone who is prone to stuttering will stuttering (meat, fruit, veggies, no/low sugar). Unless your therapist was a stutterer and now is fluent they are guessing, the blind leading the blind. I tried it all. I’m 52 and fully overcame stuttering in my late 40s.