My guess is stuttering is due to a combo of trouble formulating sentences; coordinating tongue, lips, vocal cords, breathing; and anxiety. It's rarely just coordination, for example. Usually anxiety and avoidance is overlaid. It increases as you try to avoid it. In his case something broke through the emotional-anxiety-over-thinking-trying to hard- barrier. Kind of "enough!" "stop it!" Maybe he had a new confidence to stop stuttering? I knew an adult who said he "stopped stuttering" in one day when he was in a university classroom full of students majoring in speech. He was up in the front giving a talk on stuttering and as he stuttered they started taunting him. He got really angry and lashed out---of course being perfectly fluent. The taunting had been planned ahead of time. His anger and subsequent fluency transformed him at a gut level somehow. He didn't stutter after that.