What is your preferred way for a non-stutterer to respond to a long stammer?

What is your preferred way for a non-stutterer to respond to a long stammer? I’m on mobile, sorry if the format is weird. I have this customer who is always so sweet, also he’s super cute but that’s BESIDE THE POINT. He has a prominent stutter, and it often takes him a minute to finish saying “Thank you” I always appreciate the effort that he puts into being polite, especially since in between he usually gets frustrated, huffs and then goes back to continuing the word he left off at. I’ve never interacted with someone who has such an intense stutter, though. There’s usually a large gap in conversation while I wait for him to finish what he’s saying, and sometimes it can feel a little awkward. I know, for the most part, what he’s trying to say (since we only exchange very common pleasantries) so is it rude to finish the sentence for him? Or should I wait in silence like I have been doing? Maybe tell him to take his time since he gets frustrated (what if he thinks it’s patronizing??) Also please correct me if any of this sounds rude or is worded insensitively or poorly.