"No 13 year old should be seeing that!"
Right, but OP can't stop him seeing any of that. She can only stop him accessing it on his own phone. Friends will still be showing him stuff at home, when hanging out. Surely the ideal is, if he does see something upsetting, his parents should be a safe space to talk about it. Whereas as it is, OP will be the last person he will tell, because he knows she will say 'Well you shouldn't have been looking at it,' or 'Well that's another reason why you definitely won't be allowed SC.'
I actually agree that in an ideal world kids shouldn't have access to those types of SM at a young age and if the govt did manage to bring in restrictions, great. But currently, when most kids do have access, to me the disadvantage of being the one left out, to the point it is upsetting him that much, is greater, as that is a consequence that is currently actually happening, rather than a potential that might happen if he gets it.
TBH even if the govt did bring in restrictions, it's likely it might be limited to 13 plus anyway, as that's supposed to be the current age for both SC and TT, so 'government banning it' wouldn't make a difference to OP's issue.