@Greenfrost I'd suggest you show him the site, if only to prevent him from finding malware on other sites and potentially corrupting his phone. You can also help him make an account and explain the filters (and talk them through with him, explain the purposes). He can choose whether to block or not block certain types of them, but more than likely if you bring it up yourself, it'll be addressed and he won't be interested. If he is, he can do it, but that way he knows what he's getting into. The filters on this site and its tags are probably one of the most important parts, as other sites tend to not have them, (as they aren't user-updatable).
I would maybe mention to him that some of the people on the site may be rather unsavory, depending on where he looks, but also remind him that there's the report function, and that he's welcome to use it. Making sure he knows that they're there to look out for, but also to make it clear that it's not something to be emulated would be the best option, I think.
Parents/caretakers who try to keep their child "free" from everything tend to make it so that when they inevitably encounter it (the caretakers won't be there 100% of the time in their life, irl or online) they won't know how to deal with it. Talking openly about it can help him know that he (1) won't be punished for bringing it up, (2) can talk to you about it—esp if there were parts that disturbed him, or even just things he's interested in, (3) can use things to make his user experience better/filter some things.
Don't track him or his use of this site, as that just leads to kids becoming more sneaky, and often times reading that kind of stuff for the sake of "rebellion," if unconscious. Instead, leave him to his own devices but recommend ways to make it more palatable, such as suggestion filtering out the Sexual Assault / Smut tags / whatever other tags he may just not be interested in.
I wouldn't be surprised if he's already consumed some of this kinds of media, but at least this way he can… well, what I said above. And I think it'll help your guys' relationship, too, by fostering openness. Don't police what he reads, as that'll just lead to being more sneaky, just give him an avenue for support.
(Edit: To correct a previous commenter, aggregators are generally more harmful, as there aren't warnings, no way to filter things out, and many of them focus solely on hentai/gorey content. They also don't tend to report users who may talk harmfully, and even like… the spoilers on other sites is just a pain. The Mods here, whatever people think of them personally, are certainly adept at responding to reports and are clearly doing their best to make it a safe(r) place, whereas there are generally no consequences for racial slurs, etc. on other sites.)