It could also be an attempt to teach the son about different genders. By dressing him as a daughter, the mom might be trying to show him that there are no strict boundaries in terms of clothing and appearance when it comes to gender. For example, in modern society, we are more accepting of boys wearing dresses or girls wearing pants. It's a way to break gender stereotypes from an early age.
The son might have been confused at first. Dressing in a way that is not typical for his gender can be a strange experience for a child.
One reason could be for fun. Maybe it was a special occasion like a family photo shoot where she wanted to have some unique pictures. Another reason might be to teach her son about gender equality and breaking gender stereotypes by showing that anyone can wear different types of clothes.
Well, in a story, it could be that mom was reminiscing about her own childhood and she had always wanted a daughter. So, she dressed her son as a girl to fulfill that little dream of hers for a short while. It could also be that she was trying to teach me about gender equality in a very unique way, by making me experience what it's like to be a girl for a day. Maybe she also wanted to show off to her friends how adaptable her child was.
One possible reason could be that the mom has a strong love for the concept of having a daughter and is projecting that onto her son. Maybe she always dreamed of having a daughter to do girly things with, like dressing up or going to tea parties, and she thinks forcing her son into this 'daughter' role will fulfill that unmet desire.
It could be that he has some trust issues. He might feel like he's not being told the whole truth about certain things, so he resorted to spying to get the information. Another reason could be that he saw others doing similar things on TV or in stories and thought it was okay to spy in real life.
One reason could be a total breakdown of family order, like if the son has some serious mental issues that make him unable to understand proper behavior. Another could be extreme anger or resentment due to some past trauma within the family that has made him act out in this unacceptable way.
It could be a misunderstanding. Maybe he was just using it for some art project at school where he needed different fabrics and the panties were just one of them.
One reason could be to help him with a bad habit. For example, if the son bites his nails, the mom might hypnotize him to stop this behavior. Hypnosis can plant a subconscious suggestion to break the habit.
One reason could be to shock or break taboos for the sake of creating a controversial story. Another might be to explore the idea of inappropriate boundaries in a very extreme way, perhaps in a fictional or satirical context. However, it's not a common or acceptable topic in normal family - related stories.
He might have been excited. Kids are often open - minded and enjoy new experiences. He could have thought it was cool to wear different clothes and look different for a change.