People who stutter (PWS) aren't monoliths so take your cue from him. Does he flinch or look annoyed when he stutters, or does he just roll with it? If he seems unflustered, chances are good that he's comfortable enough with his stutter that you can probably straight-up ask him how to be the best listener/conversational partner for him. I generally don't like it when someone fills in my words for me, though if it's a word I'm really stuck on, I don't mind a little help (my husband is awesome at knowing when I need a pinch hitter) just for the sake of not disrupting the flow of conversation or forgetting what my point is. I NEVER like it when people look shocked by my stutter. That's the worst for me, when someone's eyes widen or they start smirking or they get that "Oh, you poor thing" expression. Mostly, though, not being treated any differently than anyone else is what matters most to me. I am more than happy to talk about my stutter and I wish it weren't so taboo to ask about, since I'm big on everyone communicating to best advocate for their needs. Thanks for asking! It's really awesome that you want to be the person your coworker needs.