If he's a regular and it's obvi what he wants... You can't help but know what he is about to say. I think well meaning people do this and really mean well: the fact that you came here to ask shows that. I actually love when people ask me things like this but I don't suggest you ask him. For me it's validating but might not be for everyone. Then it's like there's nothing awkward in the air and I almost always feel at ease next time I see that customer. Some of these people also call me by my name and are friendly without being condescending. If you ever finish a word by accident I guess you could ask. Maybe the people who asked me just did so because they caught themselves trying to help! I don't represent all of us but you can never know what happened to us earlier in the day maybe someone very rude finished my sentence earlier or even imitated me. it's just not as easy to let it slide when a well meaning person does it on a day I've already been put off. Famously a woman let's call her Hortence comes in and asks what I'm gonna say today and that she's gonna try not to laugh but she can't help it yadda yadda. She obvi isn't senile. Hortence looks visibly confused when I have a fluent interaction with her. If she's in line I get her stuff ready so she can hurry up, shut the up and get out of my sight. Regulars at my job interrupt me all the time but the interaction is so mechanical anyway that I don't care. It's hard to explain but when people finish my words and it feels like they are rushing me that's the worst. If I'm really stuck and I feel familiar enough somewhere or with a staff member I've seen more than once it won't ruin my day. I have sweet aunties who come in and there will be a wicked long line and they'll see me getting anxious and say "sweetie don't let anyone rush you, take your time!" And they will be interrupting me to say it but they are being so sincere and validating when they don't really have to! Refreshing! If it makes you feel awkward I think the best thing you could do is nod naturally, sort of in agreement while he's finishing up. I usually don't make eye contact when really stuck but when I'm done I'm looking for those eyes again so try not to be uncomfortable if a stutterer looks uncomfortable speaking: we manage and if your concern is genuine personally you wouldn't ruin my day. I love leaving tips for helpful fluent people here sorry if this is long. So many people don't ask so I assume barely anyone wonders or cares! With family I stutter less but they know me so well that honestly when I get stuck by that point they get what I'm saying and before I'm through saying "wanna get pizza" they'll start saying "sure, from Mario's or Luigi's?" And they somehow have heard me without speaking over me or rushing me. I don't think I could give strangers instructions on how to do that. Stuttering for me is this thing where if I was by myself or only with family most of my life I don't think my stuttering would be half as bad. But it's a big world out there and... Now I want a damn sweet tea!