Well if it was me i'd use slow psychological torture on the nerd. Say things like, "I wonder how far the blood would squirt if I chopped off your fingers." "Did you know that lime will get rid of abody really fast." Everytime you are holding something sharp look right at him with a big grin (do nothing that could otherwise be considered threatening). No direct threats, unless you are alone. Nothing normal for threats either. Don't say I'm gonna beat your ass. Try if I cut you into pieces you'd just fit into my locker, I measured. The day before bring a tape measure to school and make it apparent that you are trying to measure something. Just remember don't say this in front of anyone but you. That way you can deny everything.
If that doesn't work get a rumor out that he has a tiny unit.