I have a 7 year old DD, a 5 year old DS and a 3 year old DS, and none of mine have ever done anything like that.
Maybe he just finds it odd that your DD's genitals are different to his, and he's only just realised it?
I dunno...are you the sort of family that is comfortable with nudity around each other? Or do you mostly keep yourselves covered up in front of each other?
In our house we're fine with nudity around each other, so none of the kids consider it strange that boys and girls look different, because they're quite used to seeing each other (and me and DH) and they know that "girls don't have todgers, whereas boys do". It's just normal. Nothing to look at it because it's nothing they haven't seen a ton of times before. We get changed in front of each other - the kids run around naked in and out of the bath to the bedroom when it's bath-night, etc. No big deal.
But if your DS isn't used to seeing girly bits, maybe he has only just clicked on that there's something different there, so he's curious about it?