Even if the experts don't know anything, it's just common courtesy to know its rude to interrupt someone while they're speaking. You also shouldn't need to fight for him to respect you, or give him data to prove it to him. Even if everybody who stutters thought it's fine to finish our sentences, except for you, that wouldn't make it okay. If it bothers you, he should care and respect that. Maybe just say "Please don't interrupt me" and continue with your sentence. If you can sit down with him and tell him in terms of "when you do this, I feel like this". So, "When you interrupt me while I'm having a block, it makes me feel disrespected/embarrassed/insecure/whatever." Come at in from an angle of trying to get him to understand. Instead of just saying it bothers you, tell him why. How it makes you feel, tell him that now even just speaking to him makes you nervous because of it. Explain it's a struggle you have that he doesn't understand, and that you'd appreciate if he made an effort to he more understanding and accommodating.