There is certainly some depraved content out there and Internet bullying too so I understand you wanting to protect him and avoid corruption of his innocence, but keep a couple of things in mind
He is not innocent and has natural curiosity
His morals have already been shaped by your actions over the last 12+ years
He will want to test and set his own moral boundaries, getting caught and embarrassed is counterproductive
Invading his privacy is something you should very seriously think about not doing - rather like reading his diary
How old is he and what content actually is OK
How prudish/open minded are you and are you willing to know more about the disgusting cesspool that exists out there
How much time and energy are you willing to invest, this time might be better spent on positive family experiences and real activities like sport, music, drama, rock climbing, paintball etc.
Which is more true
You have raised an obnoxious, secretive, morally corrupt child OR you as a parent are scared silly that the Internet will turn him into one?
Either way at some point you have to start to trust him (perhaps the best thing is to explain your fears and let him know that you can observe and restrict his actions but will not do so unless there is good reason - this is something I said to my kids at 15yo after using parental controls to restrict content and hours/day through their early teens)
It's not just content. Don't forget the social aspect and that your child can just as easily be the one doing the manipulating
Anyway
The easiest option for the non technically inclined parent is to allow no devices in private rooms. Keep the computer in a common area and install a simple camera that can be viewed over the Internet and has a good view of the whole screen (something like
IZON: Stem Innovation)
Parental controls can be used to track and limit web browsing, but cannot observe files bought home from a friends house on an external drive
Don't forget to disable or track the guest user account
If he has an iOS device he cannot be observed or tracked anywhere near as easily and will naturally use one of these devices if restricted (this shouldn't allow you to cripple him by not buying one - carrying a dumb phone is the modern equivalent of too short, home-made hand me downs)
(I'm certain other manufactures care little about this since their operating systems are usually Swiss cheese even before any kind of jailbreaking and any restrictions that might exist could perhaps easily be worked around by a smart kid)
Parental controls on iOS amount to disabling the browser and the App Store altogether
Learn about screen sharing which allows you to observe (secretly if you wish) a Mac or PC while it is being used (from anywhere in the world if you are smart enough)
If I was your teen wanting to bypass parental control I would simply get myself an external drive, install a complete OS on it and boot from that whenever I wanted privacy and I would resent you for making me do it OR I would spend a lot more time with that unsavory friend whose parents don't seem to have any of these concerns