I don’t stutter (my partner does), but I would suggest just talking to this person about it. He knows he stutters, he signed up for debate voluntarily, and he knows debate involves speaking. So I’d just straight up ask him how he wants to handle things. And I know that feels like a super-awkward thing to do, but it’s probably going to be less awkward in the long run to just address it up front. Even just something like, “We’re so glad you are doing debate with us, and I really want you to enjoy this, so is there anything we should know or do to help make that happen?”