I don't have rules about what they say to each other. I would find an attempt to impose such rules to be pointless control-freakery. I only make rules about what they say in my presence. And they must be respectful to adults, too, in general.
If he were mine I would try to lean on him by saying that anything he says in print (text, online, etc.) could easily be thrown back in his face later. Maybe decades later. Treat it as a permanent public statement. He might as well publish it in the newspaper. There is NO such thing as privacy in electronic media. I'd be trying to get him to think about what he says and does, rather than me trying to ban or control it. He has to find the self-control.
It may be normal in his social group, but that's no reason for him to do foolish things, too. He has to find his own good judgement (will make many mistakes along the way, of course).