Mild Stutterer here - I would leave it up to person to tell them. You mentioned that you spoke with him for an hour about a project and he didn't stutter until the very end. I doubt people will be talking with him directly for that long so they may not even realize he stutters. Most of my team was surprised that I stutter when I told them. Your team might treat them differently right off the bat when it may be unnecessary. He might stutter at first because he's nervous, but as he settles down I'm sure it'll decrease.