When meeting a stutterer for the first time is it more polite to finish the person's word and/or sentence for her/him, or to wait patiently until s/he gets it out.

When meeting a stutterer for the first time is it more polite to finish the person's word and/or sentence for her/him, or to wait patiently until s/he gets it out. Sorry, if this has been asked before. I was buying some things in an auto parts store. The gentleman helping me was very soft-spoken and had a low voice. I asked him if he could speak up as I am hard-of-hearing in one ear. When he did speak up, I realized he had a stutter. Through the course of our interaction, it seemed to get more pronounced. (No pun intended, but I'll leave it in.) I ended up trying it both ways. Sometimes I would let him know that I understood what he was saying by finishing his word for him. Other times I would opt for being respectful like I had all the time in the world. I didn't think it was okay to ask him because I don't think it's okay to just ask people about their disabilities out of the blue.