You have good reason to be protective, so it's completely understandable. I'm assuming he's younger, so just warn him that it isn't safe to give out personal information as such. Tell him to be careful, and try to keep an eye on him when you can. But also remember, you can't protect him from everything. He must lead his own life. c:
And don't do anything violent towards him, it will just escalate the issue to an unneeded level. Good luck. :D