Hi M. B.
My son is 14 and I keep parental controls on the computer and YouTube is a blocked website. As are many others. I explained to him that there is just too much stuff that he doesn't need to see and it is my job to "police" those sites for him. I mean really, you can type "funny cat videos" into YouTube and get breasts!
Your son is normal and really not too young to begin showing this type of curiosity. My boy is actually was little behind the curve because I have been sheltering him from some things. But, mine, like yours, hears about these things from friends. So we talk about it - what his friends are viewing, why they want to see it, what is appropriate for their age, and what is respectful to others. When his friends come over they know that YouTube is blocked and certain TV shows are "banned" and they accept the rules and I have explained why these things are not available at my house. One of friends, way advanced for their age, actually told me that he never thought of it that way and it now better "policing" himself on YouTube.
Open communication is key here - don't censure him for looking - give him you honest opinion about these types of videos - an opinion from a woman - why they are disrespectful to women and why you do not want him to view them. Understand that an interest in the opposite sexes bodies is normal but teach him boundaries. I have done all that - plus I just blocked the websites. LOL
You are on the right path with your response to the videos. These are not the easiest years in parenting, but I think they are the best years for building an open, honest, bond with our children.