First, it's important to have an open and honest conversation with your son. Let him know that such stories are not appropriate for his age. Explain the values of respect and healthy relationships.
First, stay calm. Have an open and honest conversation with him about appropriate reading materials. Explain the inappropriateness of such content at his age. You could also limit his access to unfiltered media sources and encourage him to read more positive and educational books.
Teach him about empathy. Let him understand how others feel and how important it is to consider the feelings of a potential partner. You can use examples from books or movies (appropriate ones, of course) that illustrate healthy relationships based on mutual understanding, trust, and shared values.
It's unacceptable. This kind of content is not only morally wrong but also can be psychologically disturbing. Family relationships are supposed to be based on love, respect and appropriate boundaries, not on any form of improper or immoral themes.
No, it is completely unethical. A mother - son relationship is based on respect, love and propriety. Sharing erotic content, even in the form of a 'trick', violates the boundaries of a healthy family relationship and is disrespectful to the mother.