Best thing you can do is ask. When I was younger, I hated when people would finish it for me. Now that I’m older, I love it when people get what I’m saying by finishing my word - because that means I get to move on to the next sentence. If they don’t finish it for me, I actually get more self conscious because now my stutter seems more noticeable LOL. So, it really depends on the person. Just ask him how he would like for you to behave around his stuttering, which is letting him finish it on his own or you finishing the word for him.