TruelyRose, you don't need to feel guilty about not knowing what your "friend" was doing with your son. What is important is that you took action once you found out about it and went to the police. One thing I would add is that if you haven't looked into counseling for your son then you might want to consider that.

The summer between kindergarten and first grade I was molested. I told my mother. Her only action? She told me to stay away from him. She never addressed it again. Now that is something to be ashamed of.