Any talk involving sex with a teenager is going to involve embarrassment. We have to put 'don't want to embarrass' aside and accept it's going to be awkward and embarrassing. There isn't anything wrong with those feelings, they don't automatically lead to shame and secretiveness.
With my DS1, it was him asking me to help him fix his phone at 13, it was just going so slow and kept acting weird -- and I ended up asking if he was aware he had over 25 tabs of DeviantArt open and that's probably part of the issue. Oh the fanart I saw that day, I had to show up how to close mobile tabs.
We had a lot of good if very awkward, embarrassing talks as we hit a few more bumps over the next few months with online content, more with graphic novels that ended up upsetting him but there was one incident with a porn site, and eventually he basically said that he'd prefer not to have that access at all, it was just too much - yes he understood his curiosity was natural but what he found trying raised far more questions than answers and ended up weighing on his mind, so we chose to remove the browser from his phone and he only used our main computer which is attached to a large TV for the next few years. It was like a weight was lifted from him.
We talked about it over the years, what he calls 'very thorough' ones, and he would repeat that he didn't feel ready to try again yet. It wasn't until he needed a browser in college classes that he asked for it to be unlocked, and even then he wanted limits on.
So, yeah, I think it's great that you've blocked pornhub, and this can lead to very awkward, embarrassing, but useful conversations to help him figure things out. Sure, curiosity is natural, but there is nothing natural or gentle about the internet's way of addressing that curiosity.