What if I said nothing and he ended up screwing the neighbour's three-year old, in spite of his protestations that he would do no such thing? I'd rather have his ostracisation on my conscience than that.

I'd rather respect his right to privacy than play "what if?" games. Ruining someone's reputation/socially ostracizing them/putting myself in the line of fire for potential slander or harassment lawsuits isn't something I want on my conscience.

I encourage you to consider the question posed by others in this thread: where do you draw the line with hypotheticals?

he has told you that he would like to fuck children. That isn't speculation. You're saying that if you worked at a job in which you were responsible for working with young children, and a coworker told you he wished he could have sx with them, you wopuld not think that was any cause for alarm or anything worth reporting to your employer? Becaus if that's the acse, then you should be fired too.

Maybe, but I'm not employed by the Mind Police, and I won't risk ruining someone's reputation, or inciting potential harassment or finger-pointing, over what they wish was legal without the intent to commit a crime.