I think you should show attentiveness in whatever way you normally would if you were speaking to a non stutterer. So as much eye contact as would feel normal for you. As long as you’re still showing them you are listening! The main thing to not do is finish their sentences for them. People who stutter for the most part are basically chronically obsessed with trying not to stutter and worrying what people think, so the best thing you can do is show them that it’s totally not a big deal.