Sexual exploration is normal, yes. But based on my own experiences, this is going beyond normal exploration. My 2 boys were both sexually molested by older boys on separate occassions. I've seen the resulting behavior. Molestation happens waaaaay more than people want to believe. Our unfortunate experiences spurred discussions with other adults, professionals, friends, and we're shocked at how many people we know that been molested themselves, or have kids that were molested. One of my son's perps had been exposured to pornography by his step-dad. At 7, your son's obsession is telling me there's a good chance either your son was molested, or had an experience with a peer that was molested or exposed to porn. I would recommend professional counseling.
This is a normal curiosity what he should learn from you is that it isnt a good idea to take his clothes off with other people but if he is alone its okay, you do not want him to feel his sexuality is a bad thing, all children do this in one from or another. Have a good chat with him and explain that and perhaps if you can be around rather than him being alone with other children till the Habit has passed .
i understand where your coming from, my son is 7 and i am having alot of behaviour problems with him.. i was told on a few occasions that my son had pulled his pants down in front of children, i was so shocked and he got in huge trouble from school, but he also told me that he did it because the kids thought it was funny.. i was so worried bacuase i did not want my son, to end up doing anything terrible.. maybe your son has seen these things from someone else, or picked it up on tv? i would speak to the doctor again, and to your son, and really tell him, that it is WRONG to be naked and show his privates to people. things can get much worse from here, but if you get on it now, it might settle down..