um, does the sergio street part have anything to do with the kid? it just popped out at me.
For the WIT:
say something along the lines of "i don't know where sergio is, but if you just calm down, sir, i will help you find him." then, if he puts down the knife, say, "sir, i would like to ask you a few questions." and continue with your interrogation. If he continues threatening you, pick up some type of statue or something to defend yourself while you run backwards out the door.