Metric, as in a 9MM to the back of the head. Alternatively, he could take one in each knee, one in each elbow and one in the offending organ. A day or two later, one in the head.
What's the rush?
I worked for a psychologist and according to him (and others in the field) pedophiles never "get better". There is NO amount of therapy that loses the urge. castration does NOT do anything for the drive. Putting them away for life is the ONLY solution.