Try an old fashion Victor rat trap. Looks just like the old fashion mouse traps only larger. Nail it to an outside wall of the coop about 8-12 inches off the ground with the trap set so that the jaw swings down. Bait it with some chicken liver of any small chicken part. When he goes for the bait the trap will spring and it should break his neck. I've used these for weasels before. It should also break the neck of a mink as well. Cheap, easy solution.