As a lifelong stutterer myself I don’t have the answers but I can tell you I got almost nothing from the 4 speech therapists I had throughout my public school experience. All they ever told me was to slow down and take a deep breath which never helped me. I’ve taught myself a few methods to help with not getting the words out, for instance learning to use a synonym instead of the word you are stuck on. I don’t really have any answers for you but I’d say just be supportive of your son, most people’s natural inclination is to mimic the stutter and laugh which isn’t exactly great for self esteem but that’s a natural human reaction. Helping him to become comfortable in his own skin is the greatest gift you can give to him. You seem like a great parent to be reaching out for advice and support like this.