the “grown up chat” about responsible use hasn’t worked
Take it off him if he can’t be trusted.
There are various security apps kicking about, but what real value they are I’m not sure. A website blocker probably isn’t going to stop him sending cock shots on Snapchat / WhatsApp / a.n.other chat program you’ve never heard of. A voice-only plan and switching off your Wi-Fi at night might slow him down a bit though. Look into OpenDNS also.
But if he’s tech-savvy then you’re probably on a loser looking for technical solutions. The idea of my parents being able to lock me out of my own tech is frankly laughable, at best you’re giving yourself a false sense of security. If I were your lad I’d say “sure, fill your boots” and then just factory reset it / stick a custom ROM on as soon as I was out of sight.