Had a lovely weekend away with DH only to find out that DSS (11) has set up Twitter, instagram and Facebook accounts using a newly set up alternative email account

We're so disappointed. He doesn't even know that we know yet as he's fast asleep. Really not sure how to handle it. Does anyone have any experience/suggestions? We have recently had a discussion about social media and made it clear he would need to be at least 13 for any of them.