Let me start by saying, you're asking the absolute correct questions. "How can I support him?" and not "how can I cure him?". I can speak from experience, just KNOWING that someone has my back, let me finish my sentences, don't judge me etc, make me stutter way less. My girlfriend does that. More often than not, she can look at me and already see that I am having a "can't-speak-for-shit-day" and simply asks me if I would like her to do/say/order whatever it is that we are doing. By asking, eventhough she already knows the answer, subconsciously I am taking the decision to let other speak for me, so in my mind it's not a defeat. I have good days and bad days like everyone else. Everyone is different of course, but the best support someone can give me is by not doing anything. In the scenario you described in your OP, I would like for my GF to treat me as she would anyone else and just wait for me to finish. Sometimes the struggle is real and I usually give her a certain 'look' if I need to be rescued, lol. But I think it is best if you ask him - in my experience, most stutterers like to talk about it (how weird that may sound). Simply by asking how he would like you to react is a really nice gesture I would have appreciated.