margy pretty much nailed it!
when my Df and i lived in KY, i had installed and paid for a "service" that would allow any keystrokes on my computer be emailed to me....i did this at 1st b/c we had a married couple(friends of his) living with us, and although my comp was pw protected, i knew people were using it, and df didnt know my pw....well, with the service, i was able to learn quite a few things, that also helped me gain the courage to kick them the h*ll outta my house....i had meant to unsubscribe from the svc when they left, but since my stepdaughter liked to use the 'net for disney, cartoon network etc, i left it on....she was/is younger than your son, and for me it wasn't so much seeing what she was doing, it was seeing who was talking to her, as we had aol, and even though the parental controls technically disallowed anyone over 10 yrs ttt her, you never know when someone may sign on with an underage acct intentionally.....idk if this is something you'd want to do, and if anything, you don't have to check it constantly, but you have the ability to every once in a while, see who's he talking to and what's being said to/by him.....
PS Make sure the PC is in the Living room or Den not his in the Bedroom it is harder then and Parents should see what is going on there.
You seem to have a handle on how to deal with it, its possible he has friends that look at the "Porn" and he is copying, With so much Tech stuff its hard to keep an eye on it, can you block it I think you can check it out then he wont have access to it. The lots of sports is a very good thing in his life the more the Merrier, get him worn out!!