I recently got a project at work where I need to regularly talk to a person who is suffering from very bad stuttering. He is very competent and professional at his job, but talking with him is a bit tedious, because he frequently has issues forming a complete sentence.
What are the general do's and dont's when talking to someone who is stuttering? For example, when he has trouble completing a sentence but I think I know what he wants to say, how should I act? Should I wait patiently? Should I complete the sentence for him? Should I interrupt him with my reply? What is the general etiquette for dealing with this very common condition?
My goal is to communicate efficiently with him but without hurting his feelings. But I also don't have a close relation to him, so I think it would be impolite to directly address his condition and ask for his preferences.