My new therapist has a stutter, need tips and insight!

My new therapist has a stutter, need tips and insight! Basically I was set up with a new therapist, it's mostly talk therapy. They told me when scheduling that he wants people to be aware of his stutter before they make an appt. I said it was fine, as I really don't mind and I've spoken with some people who had a stutter before. However my therapist's stutter is more.. severe and apparent than I'm used to. It seems he struggles with every other word for quite a bit of time. I still don't mind it, I like him, he's educated, he's nice and after just one session I trust him which is tough for me usually. I just want to get some info on really what a stutter is, what makes it worse in certain moments, and how I can be the most respectful to him as he is to me. Like, if I know what he's going to say is it ever okay to finish a sentence? Or answer the question that I know he's trying to ask before he finishes asking? I know I could just ask him but he seemed very insecure about it at first and kept asking if im okay with it, and saying if he needs to repeat something that I should ask him to. I just don't want to make it a major issue with him, it has nothing to do with my treatment.