It is normal for a child to "experiment" at some time. They realize they have feeling down there and don't know how to control it.
Have a talk with your son and explain to him what is right and what is wrong and tell him that even though you understand he has feeling down there it is not apropriate for him to play with "it" in public or with someone else. Also, explain the pictures are not apropriate. I bet with time this will pass.