Minimal suspicions? A young male flying alone who refuses the scanner and sets of the explosives detector?
Yes, he absolutely should have been cooperative and answered their questions after that. I always refuse the scanner, but I don't complain afterward that I have to answer questions and get a "firm" pat down (there is nothing "firm" about them, btw, they are overly careful and respectful if anything).
And if I ever set off the explosive detector, my first reaction wouldn't be how unfair it was that "harried" employees were "rudely" explaining that my options were leaving or a private pat down. That would seem utterly reasonable to me.