How do I talk to my 5 year old son about being touched inappropriately and make him understand what Im saying?

I appreciate everyone who has answered my questions about my son and step father, but one thing no one has answered for me is how to talk to him about it. I know I'm going to take him to the doctor and have him checked out but I want to talk to him about it before I take him. I tried today while I was giving him a bath but he didn't seem to understand me. I have told all of my kids that no one should ever touch their private parts, but when I try to talked to him about it he just shied away from me and then started telling me stories about fishing with mu step dad. How can I ask him in a way he would understand?