Open up a conversation with him about why he visits chat rooms, what he does while he's there, and why he used foul language. Don't interrogate him. Let him know that you want to understand his behavior and that your respect for him has not diminished because of this error in judgment.
Ask him how long he thinks he should be banned from chat rooms. Also ask him what he thinkgs should happen to his computer privileges if another such incident occurs. I also suggest you keep your computer in a common area so that all family members' computing is easily observable.