The best way to know is to ask him. Eiher way, make sure to **NOT LOSE EYE CONTACT** when he starts to stutter! Doing this will make him feel like you dont respect him, his stutter and the conversation. People doing this to me all the time, and this can **REALY** hurt. Its like people who use their phone while talking to them, but much worse. Stutters often have bad relationship with their stutter, and this act will further ingrain to them how much it sucks to have stutter.