I think it's just he is at a curious age. Sexual images and the like are everywhere so even though you haven't told him about sex he's still being exposed to it. I think you have done the right thing explaining it to him but I wouldn't make too big of a deal out of it. I do not have kids that age so I can't really offer much help but I remember when I was young around 7 or 8 I was very curious about sex and so were my cousins. I remember my boy cousin wanting to play doctor. Plus this is really TMI but even at such a young age I do remember touching myself. I don't know if it was curiousity or what but I think it's normal. I think it's an innocent thing; I wouldn't be worried he's a pervert! Sorry I can't help more.