Asking him matters. As a closest friend, they're likely already at serious ease around you. Even if they say it is OK, it might be a good idea to not finish the sentences each time you feel like it. From my perspective I don't really enjoy someone finishing my sentences and would rather it be avoided. I did have a college-mate who expressed that he'd be ok with me finishing his sentences as a person who stutters (PWS). I only did so occasionally but not all the time. The very fact you're concerned about keeping your friend comfortable with their stutter, is a good thing. Not every does, and some just finish our sentences without concern of their impact.