If he does it again, tell him someone from a stuttering forum said, "That's very immature behaviour from a 32 year old adult. That's the sort of behaviour a person would expect from kids still in grade school. He should look at himself at in the mirror and re-evaluate how he comes across with what he says. There are people who struggle with many things out of their control, he should be more mindful of this and learn how to put himself in others shoes."