He might feel a bit embarrassed if he is old enough to understand that diapers are for younger kids. But if he is really young, he may just feel comfortable and safe with his sister taking care of him.
He may feel a bit self - conscious at first. Wearing girl's clothes is not the norm for him. However, if his sister is positive and supportive during the process, he could gradually feel more at ease. He might also feel a sense of creativity being unleashed as he participates in this unique activity. And depending on the reactions he gets from others (if any), he could either feel more confident or a bit shy about his new appearance.
In some cases, the boy could feel angry or resentful. If it was forced upon him against his will, he may see it as an infringement of his identity. He might struggle with the idea of conforming to something that he doesn't identify with, and this could lead to conflict within the story, either with the people who made him do it or with his own self - perception.
At first, he could be angry. After all, his autonomy is being taken away. But as time passes, he may start to have mixed feelings. He could find some aspects of the girl's clothing comfortable or interesting, while still being resentful of the situation. He might also start to empathize more with girls if he experiences some of the things they do while dressed as one, like the way people interact with him differently or the types of comments he gets.
He may initially feel a sense of shock. It's unexpected to be forced into such a situation. And as time goes on, if he can't escape this forced dressing, he could feel a deep sense of helplessness. He might be constantly worried about what others will think of him and may start to withdraw from social interactions out of fear of being ridiculed.
He might feel embarrassed. Dressing as a girl goes against the traditional gender norms he is used to, and he may be worried about what others will think of him. He could also feel angry that his will is being violated and he is being forced into something he doesn't want to do.
A 'broken boy novel' typically refers to a type of novel that focuses on a male character who is 'broken' in some way. This could mean he has endured trauma, has emotional or psychological issues, like a boy who has grown up in an abusive family or has faced a great loss at a young age.