First off, good question. Seriously. You are right that one size does not fit all, so I will say that what you can/should say might depend on where he is on his stuttering journey. By that I mean he may be hiding his stutter because he has yet to come to terms with it. It may be quite embarrassing for him at this point. But, it may not. Maybe he is in a better place with it and he is just not sure how to proceed or not sure how other people will react to his stutter. The fact that you want to hear more from him because you like his work and he has good things to say is a great jumping off point. I think by starting with a good compliment you set the table well before you dig in. I'd do it over chat (private of course) and just let him know that you have his back. Lastly, make it just between you and him, not the team and him. If its the whole team and him, he may feel like he is in a fishbowl, if you catch what I throw. Make it seem like a small thing, then go from his reaction. Good luck.