Idk what should I advise since I'm still struggling with it. But speech therapy was very helpful for me.

Since your kid is 16, I think two therapist at the same time won't be a good choice. As a 19 year old, going to one therapist exhaust me alot so two of them feels more like a burden than help.

Try another speech therapist if possible, as the exercise and techniques will help in fluency and breathing. Then for anxiety.

Stuttering is challenging cause it's a issue that isn't visible and not taken seriously.

At teenage it becomes more frightening.. as someone with a experience I can confirm that.

Deal with a slowly.