How do i teach my four year old that it's not ok to look or touch other peoples ual organs

About a year ago when my son was 3 1/2 years old he spent a weekend with his father and something occurred that left me appalled. My x husband called to tell me that he caught his then four year old nephew and our son with their pants down. They spoke with his nephew and discovered that he was being ually abused by a family member. Now my son had been exposed to a level of ual curiosity i have never encountered. I explained to him how he should only explore his body in private and never look or touch others. I confronted my x husband and his family about the situation and was reured that the situation would be addressed and something would be done about it. Just recently, my now four year old son told me that him and his cousin still touch each other from time to time. He has also showed an interest in asking his step father to take his pants of so he can pull down his boxers and look at his backside. I am in the process of having my son removed from his cousins company at all times but don't know how to address the situation further without him feeling too ashamed or embarssed......