First, you should try to get away from the situation as soon as possible. Then, find someone you trust, like a teacher or another family member (preferably not related to the abuser), and tell them what has happened.
The status of manhwa hentai is quite controversial. It's not widely accepted in many communities due to its explicit and potentially offensive nature. Also, it may violate certain laws and regulations depending on the jurisdiction.
First of all, it's important to stay calm. This is an accidental and embarrassing situation for both of you. Try to act as normal as possible and avoid making a big fuss about it. Maybe just quickly look away and pretend it didn't happen. If you feel really uncomfortable, you can later on find a time to have a private and honest conversation with your brother, just briefly mention that you hope this kind of accident won't happen again and that you both should respect each other's privacy more.
One reason it stands out is the detailed world-building and the unexpected turns in the plot. Also, the character development is really well done, making you root for the protagonist throughout the story.
First, stay calm. Apologize to your son for the situation. Then, have a serious talk with your friends about respecting privacy. Make sure your son understands it was an accident and that he has nothing to be ashamed of.