Hi,
Your son has made a good first step; not watching porn, but feeling guilty about it and telling you about it.
Here is a link:
http://www2.oprah.com/tows/pastshows..._menpoll.jhtml
You might tell your son to look at the above link by Oprah Winfrey, which is a survey. It might convince him that this is a National Problem, and is causing problems with marriages and with boyfriends and girlfriends.
Watching porn can also become addictive....just like any other addiction. Those types of persons even print out porn pictures, carry them around school, showing them to others!
Another study, announced on National Network TV news, shows that watching porn can actually lessen the sex act with a spouse, because one's perception of sex is heightened so much, that they feel "let down" when actually having sex. But, a 13 yr old might not understand that just yet.
You are definitely doing the right thing in talking with him about it, and I am sure some others will be along and give you some suggestions about what to say to him. I do wish you the best, and just keep talking with him, and encouraging him to stay away from it. It's all over the web now, and even doing a search on google.com, or other search sites, for anything, can even show links to porn sites!