I found out my teenager is mocking or bullying others online. How do I deal with it?

The correction here must be firm about the behaviour but must not shame your child, so that he learns responsibility, not just fear. Allah forbids mockery: “let not a group of people ridicule other people” (Surah al-Hujurat 49:11). Sit with your child and help him imagine the feelings of the person he bullied; empathy changes more than punishment alone. Ask him to delete the content and, where appropriate, apologise to the victim. Explain that what is written online leaves a lasting mark and can have consequences. Check whether peer pressure or another problem is driving him. Monitor with love while rebuilding his character. See educating the family.