i.e. "letting him know this isn't normal for a 10 yr old".

In all likelihood, he stumbled into the porn rather than searching for it directly. People make porn out of all kinds of characters so a kid searching for their favorite cartoon might accidentally find a video of their favorite characters participating in a gangbang.

I don't have a child that age but my thinking is that now that the kid has been exposed to porn, if he wasn't already curious about sex, he likely is now. If that's the case, it's better to be a reliable, mature source of information than to shame them into finding their answers elsewhere (like on the internet or from equally ignorant friends their own age). They're still going to be curious, you just want them to go to the right people with their curiosity.

*edit* NVMD on the parents thing... it's up to you have that convo. The kid may not be down too far the rabbit hole of porn so talking about it now may not be too difficult.