Here's a tip I often use for myself. When my argument relies upon "everyone being stupid". I really should take a second look at my argument.
And I agree, working with someone so cocky is a bad idea. Because if you ever do follow his advice (against your better judgment) and the projects fails (as it likely will), it won't be because of his idea, but its because you were stupid too.
I don't think that strong typing is bad, but I would like to point out that from his point of view, arguments FOR strong typing are based on the premise that "everyone is stupid."
Orthogonally, you don't have assume that everyone is stupid for it to be a good idea to design things as though everyone is stupid, because the fractional slice of attention and working memory that the user can easily spare is effectively a moron. This argument applies easy to issues of usability, but for programming, it may equate to driving a car with blinders on. I don't really know.
From my point of view, the main arguments for strong typing are that everyone makes mistakes, and that your tests might not be as complete as you would like (due to time pressure, or whatever else).
And I agree, working with someone so cocky is a bad idea. Because if you ever do follow his advice (against your better judgment) and the projects fails (as it likely will), it won't be because of his idea, but its because you were stupid too.