I agree with everyone else that there is no universal rule. I would just ask; I'm sure he would appreciate it. I personally like when someone just waits and maintains attention while I take a breath and reset. I am not one that appreciates someone finishing my sentences. I understand why some stutterers appreciate this, but I have such bad experiences from elementary school where teachers and especially students would not have patience for me. It makes me feel like their time is too important to wait for me to get my words out and what I have to say doesn't mean anything. This rarely happens as an adult, but a coworker or two have done this. Sometimes it is also helpful for the listener to remind me to take my time, but mainly only family members will do this. It's all about what the speaker prefers. It's really cool of you to be aware of this.