My 7 yr old stepson does this too. I've known him since he was just 2, and we have full custody. He doesn't seem to mind running in front of me buck naked, but when it comes to Dad, he says "I need privacy!", he also likes when I sit in the bathroom with him and chat while he showers. I think he's starting to realize he's a boy, and he and his Dad have the same parts, but for me this does not apply, so he doesn't feel the need to be modest. I have no idea! I'd give your son a bit more privacy and see how he responds to it.