When I was 15, I discovered Usenet and the various binaries subgroups. This was beyond what my folks understood was on the PC. Your son will access content beyond your current understanding as well. Additionally, you might have a monitor on the PCs you own, but you don't have a monitor on his friends PCs either. The monitoring game is already lost. You indicate a loss of trust, but it goes both ways. There's absolutely no way he's going to tell you anything now that you've dealt the monitoring card. He's not going to thank you in the future. He's simply not going to say anything about it.