I think the most effective tool is communication. You can take more practical steps to blocking porn from your computer (like setting up a parental filter, just make sure it's a good one he can't bypass, or supervising his online time) but he may still find ways to acquire that material or watch it with your knowledge so it's important to equip him with the knowledge necessary to make good decisions for himself and to deal respectfully with any future sexual partners he may have.
Some pointers on things you should mention if you talk to him. You can explain the practical problems of viewing porn online. Not only are lots of site full of viruses or are paid sites, but they can also include material that was put there without the consent of the people depicted in it. Not only that but if he's looking at porn with people around his age in it, he may see nothing wrong with that but it's considered underage porn and can get the adults in the household in trouble.
You should also explain that porn is often very unrealistic. Just as with action movies where the main character is shot several times in a row and can still go around punching bad guys, porn often manipulates the images and can lead to unrealistic expectations about sex, about how he or his future partners should look like, and even how to behave when having sex with someone else. For example, lots of porn is very degrading to women, and even though some people are into that in real life, that's something that should always be negotiated between the two people involved to make sure no boundaries are being crossed and that both people are enjoying what they're doing.
In general, being open to talk to him about sex and how to stay safe and keep his partners safe (like preventing STIs and unwanted pregnancies) and on respecting the boundaries of others and not doing anything against their consent (there's lots of material online on educating teenagers about consent and negotiating boundaries with sexual partners, you can research that, as much of it is geared to teachers and parents who have to teach sex
ed. stuff) is very important. Do your own research on those subjects if you have to. I know lots of guys don't identify with feminism but even if that's the case with you, don't dismiss feminist-leaning sex
ed. materials as many of them discuss very important topics like consent and sexual health, especially if his partners in the future are women and it's an important perspective to consider that you may not have yourself.
Finally, if you're unsure whether the time is right to discuss it with him, you can just make sure he knows he can talk to you about anything and ask any questions he may have, and just giving him a general talk related to puberty and the changes it involves can help him be prepared for it and if he asks any questions or seems to want to ask them, you can help him and explain about those subjects.