S.O.
Mine did this too. I just began calmly removing his hand and telling him "No. That bothers Mama." Then, if he tried again just because he knew it bothered me, a firm no with a firm removal of or a swat to the hand followed. He got the message and hasn't tried to do it in a long while. It's also a good excuse to have the conversation about private areas of the body and good touching vs. bad touching. It's so important they know this at any age. Some areas are just off limits no matter how innocent the intention is.