Some people like when others finish their sentence, but ultimately it will not be the best for his development if he just stops talking, gives up, or has someone else finish his sentence. Even if he wants it. I believe the best possible thing you could do, is tell him what you wrote here. Make very clear to him that he's a valued member of the team and his input is needed. And make very clear that it doesn't affect you guys and the information he shares does not get diluted if he stutters. Showing him encouragement, appreciation, and patience even when he doesn't want it, and is struggling, Will be massive for him