I need advice on how to behave around my stuttering friend.

I need advice on how to behave around my stuttering friend. Hey you guys! I don't stutter myself but I've recently been developing a closer friendship with a really great guy who does. He's rather open about it, like he has a mantra hanging in his room that he explained to me and he answers questions I have about stuttering. I don't have a problem with it at all, we have really great conversations and most of the time I don't even notice it anymore. But from time to time it happens that it takes him a really long time to say a word and then I see his frustration about it and I'm not sure what to do. I know that I shouldn't finish the word or sentence for him because that's disrespectful and whenever someone does it, I can see that it hurts and upsets him. I usually just look at him until he gets it right. Is that the best way to react? Also, can you tell me what could be situations and things that influence the intensity of his stutter? Is there something I can do or should I just let him sort it out on his own? Do you have any general advice on being a good friend to someone who stutters? I would ask him directly, but I'm not sure exactly how comfortable he is with me asking so many questions. Looking forward to your answers!