Offensive content – pornography, extreme violence, and other anti life material are all available on the web.
Predatory or harmful people Sometimes they form unhealthy relationships with those they meet online
Disconnection from real life Even when the information and connections are innocuous, teens can become overinvolved online. They can spend too much time on the computer and not attend to homework or other responsibilities.
If the situation is serious you may need to remove all online access from your teen. However you can set some other parameters that can help you protect your teen while still allowing access to the internet
Know what makes your teen tick – some feel disengaged from reationships (the internet fills the void)
Some have little impulse control and are drawn to harmful material
Others who are unhappy with their lives get on the internet to escape
Talk to people with computer  experience – there are all sorts of ways to block your teen out from inappropriate content or use – find out from computer techs.
Insist on safety over privacy – If he is using the internet in unhealthy ways you may need to monitor what he does, either manually or electronically .with software that can do that. Once you see improvement you then monitor less.
Require life in the real world – Insist your teen has live contacts. Set limits on when and how long your teen can be on the internet
Establish the internet as a privilege not a need