Let's just say I'm highly skeptical. First, this isn't a peer reviewed paper or anything remotely close to it. One man's anecdote is basically worthless. Second, I believe something like 2/3 of the human population is lactose intolerant (maybe not that high, but it is over half). Stuttering exists in China and Japan just as much as it does in the US and northern Europe. If this claim had any validity you wouldn't expect to see this. Third, dairy is just some ratio of carbs, fats, and proteins, same as every other food. There's nothing inherently special about it. Unless if lactose, for some strange reason, targets the speech areas of the brain, there would be no reason why dairy in particular would have an effect on stuttering.