One thing that you can do is when he is talking and you notice he is about to stutter (realize how he starts to speed up a bit) just tell him “hey your starting to stutter, can you say that over again but a bit slower? “ Us stutterers forget that when we stutter we sorta speed up mid sentence which this causes the stutter. You reminding him to slow down can be a bit calming and help retrain the brain to not go on high speed when talking. To fix a stutter is basically retraining the brain and it’s response to talking when talking. Retrain the brain. Change the reactions that occur during speech and a stutter (in my theory) will go away. Good luck.