I would like to help you with your question.
I can certainly understand your concern about your child's behavior and his exposure to pornographic type material on the internet. He is at an impressionable state in his development and you are right in wanting to be sure that he is not being exposed to things that are inappropriate for his age.
Children are very curious beings. And as you already told him, the Internet is not the source you want him to rely on to learn about sexuality. I'm glad that you were able to lock the system tight!
It does sound like you treated this episode (and the previous) in a sensitive, caring, and consistent manner....bodies are private and if you want more information come to mom and dad. I would back that up by getting him an age appropriate book about bodies. Here is a suggestion:
In one way, it is important to capitalize on his curiousity and expose him to books that you approve of. This will back up your statement that he should come to mom & dad and that you will be very happy to talk to him and answer his questions. It will also provide him with some information - hopefully what he learns will override any peer influence to "smell each other's butts".
Teaching our children about sexuality and our bodies is something that is best delivered over many years, with many small chats, and with good books to back up our teaching. And...most of all...with an open door policy that allows them to continue to ask questions without fear of being shamed.
I would not be so alarmed by the incident in the bathroom. This likely was boy play. But his pulling down his pants 3 times during nap time is more concerning. Did he explain this? Did he say where he saw that before (and therefore copied)?
As to the counseling. You do not want to make this a bigger deal than it is and in so doing bring negative attention to his curiousity. Like many childhood behaviors, attention on the negative typically leads to more negative.
What I suggest is that you monitor his behavior over the next month or so. If he makes an attempt to access any inappropriate sexual material on the internet, then consider having him see a child psychologist. As well, see how he handles a good book about the body. Does he become overly focused on body parts? Does he take down his pants around friends or neighbor children?
Let's see if satisfying his curious mind with good, healthy material and open dialogue with you and dad gets him on the right track.
I await your response.