I think you can get a good idea of how they feel inside by watching the student’s overall behavior and body language. For example, if the student only speaks when you call on him, this may mean that they are anxious to speak. But if the student volunteers to speak on his own, I think that’s a good indicator that he’s a proactive speaker and doesn’t want your help finishing his sentences. Or if he seems to be going above or beyond in any way, sitting in the front of the class, almost showing leadership-like qualities, then they may be confident to speak on their own. Especially if they look fairly relaxed, they may be a bit desensitized to their stuttering at this point. People that are real self-conscious about their stuttering tend to try to hide and not show themselves as much as possible. Does he seem like someone trying to hide from you calling on them, and speaking the bare minimum, or someone proactive in speaking? I think it’s a good rule of thumb though to let him finish his sentences. Though I also think it’s good that you feel empathetic about his stuttering, and you can ask him if he has any preferences for how he wants to speak in class.