Lucky,, count your blessings its typical normal healthy curiosity about the female form.
Believe it or not,, it really is not at all unusual for kids to try access to such materials.
LOL,, I find it amusing that he has learned,, state your story and stick to it. Personally, I wouldnt get all upset about it. I WOULD however make it plain that such materials arent allowed in your home,, but my advise,, let dad handle this one.
Thanx for the laff ;)