1.) Firstt things first. Print out the history and confront him with it. You need to hold him responsible for his actions and choices, and the first step in that is confrontation.
2.) Cut off his source. Password protect the computer and set it up to where he can only have access when you are there to watch him. Some people would argue over this being too extreme, but the simple truth is that he has violated your trust and this is a consequence of that.
3.) You need to establish an environment of accountability. Part of the reason he's done this is because he could without consequence. (Please don't take that as me saying "YOU FAILED," because that's not what I am getting at. I'm simply saying that he's taken advantage of the lack of accountability.) A good first step would be to install either a filter or some type of monitor on the computer. One good one is Covenant Eyes. It charges about $8/month, but it catches every website that is browsed along with a rating of the content, and I think it also registers when FTP programs are used. (IE, FrostWire). The great thing about this monitor is that it requires you to be logged in and the monitor open, or else your internet connection can't receive information. It can then send you an email as often as every week showing you a summary of internet activity with a rating. I believe they also have a filter available, but I've never worked with it so I can't tell you how reliable it is. The monitor itself, though, is very good and I've never noticed any slow downs on systems that have it installed.
4.) Try to find out where he was exposed to this initially. If it was a group of his friends at school, then you'll need to decide what measures are appropriate in regards to these friends. (Whether it's informing their parents, or limiting your son's interactions with them.) Again, this seems extreme, but you have to realize that these parents may not know their kids are doing this, either. Aside from that, if they have exposed your son to this, you have to question what else he would be exposed to.
5.) This may sound extreme, but it may not be a bad idea to see if counseling might be needed. Being exposed to pornography at an early age can have pretty adverse affects on a kid. (I know this from experience) In all truth, your son has seen things he shouldn't have in his mind at this age. Pornography has long been known to have a warping effect on the mind, and it is often a source of development problems in our society. Granted, your son is 13 and he may just be going out of curiosity.. but he's at an age when his hormones are kicking in. He's putting images of a fantasy world in his mind that are painting a picture to him that this is what intimacy between two people looks like, and it is damaging. At the very least, you need to have conversations with him to counter these pictures and correct it. After talking with him, you can determine if counseling is appropriate.
6.) I put this last, but this is the absolute most important thing. Make sure you do not react in a way that suggests you are appalled at HIM. Put the focus on his ACTIONS, not his identity. This is something that will likely feel very humiliating for him, so you need to make sure that you address it in a way that doesn't shut him down or make him hesitant to talk with you. While you may have varying emotions over this discovery (such as anger, shock, etc), you need to make sure you communicate and act in a way that says "Son, I am not happy with what you did, and there are consequences. But I will walk through this with you, answer your questions, and guide you in the way you need to conduct yourself."
I really hope this helps and gives you some insight on how to go from here.