I agree with you, but I want to try.
Even if you put some gatekeeper programs on the computer (Net Nanny Spectre, etc), or restrict the browser itself or within the operating itself, he can still boot up from either a CD/DVD or thumb drive into a LIVE version of Linux and nothing you did will matter.
If you go a step further and try to block the 1M+ (porn or other) domains by using a blacklist on your router, or calling your ISP to do it for you, he can still boot the computer from a LIVE version of Linux, or use a VPN or TOR from his phone or tablet.
There are also 'easter eggs' in gaming consoles, PS4 and XBOX to bypass any technical thing you try to do, and yes even from an Apple TV or Roku box, one can easily view porn, are you going to block those devices too?
Maybe the ONLY way to reduce his exposure to porn at home, is to have the computer he uses out in the living room, and where you can see the screen, and forbid cel phone and tablet use in the bedroom and/or bathroom, but now you have a social/behavioral battle rather than a technical one....another option is to shut off your home router, effectively disabling the internet at home, but a smartphone or tablet with a data connection via cel phone service will still get online...
and no matter what you do, he will still get access from his friends...at the library, etc
I know all of this from work I'd done prior to my current job, and friends and family have been asking me for advice on these kinds of topics for over 20 yrs.
Kids can even see violent, gorey acts, like beheadings and savage beatings on YouTube, and that content is also near impossible to block, and they WILL see it eventually, even if not at home. When I was a teenager, everyone was passing around VHS copies of the 'Faces Of Death' movies, and you could watch it on video players at the school library (until the teachers found out) but the damage was done because of the hype, the tapes were passed around and it did not matter...the word was out...
Kids today are way smarter then when I was a teenager. (Read "Big Brother" and "Homeland" by Cory Doctorow for an eye into this world, but great stories to read anyway)
I had no trouble getting porn mags when I was like 13-14 yrs old, and I kept them hidden since I did not want my sisters to find them, but my mother did, and she told me that it 'was normal & healthy to be curious and nudity is natural, but remember that these are FANTASY pictures, and keep them away from your sisters, or else', and I obeyed, and never had any issues...nor did I become a predator or a pervert...
The only way to block access to unwelcome content is to have zero internet and zero tv, and home-school your kids and limit their access to the outside world (effectively raising them as hermits), but that may do more harm (to them as adults) than seeing some 'normal' or 'divergent' sex acts and extreme bloody violence at a very young age...
I do not judge and cannot tell folks how to run their household nor how to raise there kids. I am not a parent now (almost was a step-parent of 2 pre-teen boys), and parents have a tough job with lots of challenges today.
I wish you good luck, whatever you do.