Not a stupid question at all! I thibk it really depends on how close you are with them. Like others have said, I don’t do much mind my close family or friends ending my sentences, but I hate strangers or people I don’t know we’ll doing it. I know people have good intentions but it makes an already frustrating situation worse. If you’re comfortable with it, I would just ask him if it’s okay or if there’s anything you can do to help him. I know I would appreciate it and it might make him less flustered when he stutters (which may help with his stuttering).