Quoting Uncle Obama's Banana:" So, congratulations! Your son just turned 13. You catch him watching porn online. What is your reaction? ... [snip!] ... BD or I had the sex talk around then. I don't see any problem with a young man masturbating. ETA: This can go for girls too."
He would have violated trust regarding use of the computer and would be punished for it. No punishment for masterbation as that is normal.
We'd have a conversation about porn but MOST importantly, I wouldnt want him going to places on the net that he isnt supposed to go.