I'd certainly talk to the director if you find the director a reasonable person. This teacher sounds like very bad news at the moment, but you say your son is happy there so I'd hesitate to move him or even move classes. This is the key thing to make sure the director understands: "I don't teach him and I don't understand how he does it, but he's taught himself." I'd go to the director saying that, and saying that you're considering moving your son but don't want to, and ask the director to have a word about the diversity that exists among children!
