To be honest, some people who stutter are unable to admit it. Some may even think they're hiding it when it's very obvious to everyone else that they're not. Giving him time to speak is the best thing you can do. If you'd like to be "encouraging" ever, like if he shuts down and appears very frustrated, you can say "I love hearing what you have to say". Focus on the content. The National Stuttering Association (westutter.org) is currently run by a spouse of a PWS (person who stutters), and the annual conference has had workshops for spouses before. I wonder if they'll do an NSA connects for spouses sometime. You can sign up for the emails or ask them about it. Maybe they've even done one and it's on their YouTube.