As a therapist with a stutter, please share everything with him that you shared here. Ask him his preference. My preference would be for my sentences to be finished, but I know everyone feels differently. It will likely mean a lot to him that you’ve put thought into it and also that he has made you feel safe and comfortable in therapy. At the same time, and as a therapist, it would make me feel uncomfortable if I knew my client was worried about me or putting my needs first in the session. I would just briefly ask him his preference and share his positive impact on you, then keep the focus on yourself. It sounds like you found a great therapist, and I am happy for you.