Just to update - I have now spoken to DS and he accessed the porn via proxys on Google. To clarify, he has maximum parental control on his iphone and cannot download apps without permission.
He did however have Google. The phone was locked down with a child's SIM (EE Safer SIM) AND parental controls all set up on the device via Apple screen time AND our home router similarly set up to block adult sites. I thought, stupidly, that this would mean that he could not access adult content.
No additional apps required and the parental filters did not and do not filter out proxies. It's not hard to find out how to access porn on a kids phone - if you don't belive me, get your kid's phone right now, google proxies and then click through. You'll be at Grannies of Hove before you know it.
I have spoken to him about the seriousness of the situation, but in a completely pornified world, I don't think he really believes me.
We have talked about porn - consent, coercion, and control. We have talked about unrealistic sex, violence in porn, distorted body image and the potential for long lasting damage to relationships and to self esteem in anyone that watch it. We have talked about safeguarding, grooming and the need to keep children safe online.
When I say "we" I mean me, of course. He's a child, he didn't want to have the conversation and I don't know how much he understood.
He has lost his phone for now, but eventually, we will give it back.
To anyone reading, please keep checking your kids phones. Belive me when I tell you that it's impossible to lock them down completely - if it were, someone in the last 4 pages of this thread would have told me how to do it. They haven't, becaues you can't.
As PPs have helpfully suggested, the only way to keep kids safe is to take their phones away completely, until they are 18.
If you are reading this and you are a parent who has removed a smart phone from a teenager permanently, please can you share what happened and how it went?