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A person who stutters does not choose to do it. He is not doing it to annoy you. It is scary and humiliating. He may benefit by seeing a psychiatrist about his anxiety. Seeing a speech therapist may also help him to control his breathing and gain better control.

When he is struggling with his words it helps when a person is supportive. Saying something like "take your time" is a positive thing that eases anxiety.

I didn't have a problem with stuttering until I was 38. It hit when I had a breakdown. The words felt broken up in my head and that's the way they came out. Stuttering and other tics are horrid. I found myself on the end of jokes until people realized that I wasn't joking. For the most part I have had professional help and my stuttering and tics have eased even gone but on bad days they are back. (I am only telling you this so you maybe you can gain a better understanding what it's is like from his side).

Encourage him to get help. Encourage him to breathe. If he does go and get help maybe you could get guidance on how you can help.

6 Tips for Speaking With Someone Who Stutters | Stuttering Foundation: A Nonprofit Organization Helping Those Who Stutter