There could also be a more serious side. If it's part of a story about self - discovery or breaking gender norms, it might show that the boy is being brave and exploring different forms of self - expression. For example, in a more liberal and accepting community, people might encourage him to be himself and not worry about traditional gender - specific clothing. In some art - related scenarios, it could be a creative statement too.
Well, some people might find it really funny at first. They could laugh because it's not a common sight to see a boy in a bikini. Maybe his friends would tease him in a friendly way.