I've a now adult son and had to deal with very similar issues. Since then I've done a lot of reading and thinking about the topic, and would have dealt with it differently.
Modern online porn is not the same as the old Playboy and Hustler mags that men in my generation had. Modern online porn is increasingly violent and degrading to women, almost all of it made from a male POV. It is highly addictive.
I think OP that your initial gut feeling that this is not good for his developing sexuality is spot on, stay with that. While you don't want, as others have said, to pass judgement on his sexual tastes, you can leave that out. But the fact he's been accessing online porn for his pleasure in private is a massive problem for his age.
There is a window in which this can be turned around. If he continues to regularly use online porn it will wire his developing sexuality to view women negatively and his sexuality will suffer. So blocking his access while at home is essential.
I know you can't control what he accesses outside of home, so that is where gentle conversations with him are important.
Search Gail Dines. Watch some of her talks and learn from her knowledge.
Armed with that knowledge you can have indepth conversations about why you need him to not access this and use it for pleasure. Why you want him to develop into a healthy young man with healthy sexuality that will allow him to have positive healthy relationships with women.
Finally, there is a lot of online porn made using trafficked girls and women. You simply cannot tell if the girls and women depicted have been trafficked or not. So regular users of porn will be using this material. That should make anyone with a moral compass very uneasy, enough for them to avoid it entirely.
Now, my views may be considered extreme by many on here. I don't care. Quite frankly, I think modern online porn literally in the palms of, and accessible to (whether or not they access it) almost all people here today, is a massive public health crisis that we need to tackle head on. It's unprecedented. Porn is the biggest sector of the internet and it's increasingly violent and degrading to girls and women. Regular use affects how boys and men view girls and women. We need to acknowledge how addictive it is and act accordingly. I will make no apologies for this view. I wish more people would take it seriously. I think we're sleepwalking into disaster, if not there already.