Take it all away. No more. This means get it out of the house and stored somewhere else. He cannot control himself, so no more available.. This includes everything except the TV with BASIC cable.
Remember we all lived without it? Books, Swimming, riding bikes, Volunteering, making money baby sitting, mowing lawns. If he has to have a cell phone.. no text available.
You need to show him you are serious. You also need to speak honestly with him about sex. What it is, responsibilities and age appropriateness,..
And why these are dangerous behaviors when he goes on line to these sites, these conversations with friends and strangers.. Talk with him about the laws of your community.. He could end up in tremendous legal trouble.. Or it all could also be misunderstood and others around him could also suffer lawsuits and legal problems..
He is obviously just not mature enough to be trusted.. sorry for him, but you are the parent, you cut him off from all of this..