1. All black stands for silence and composure. It may be that he has encountered some setbacks or things recently and needs to calm down for a while. Think about your life and direction. During this period, I don't want to have too much communication with the outside world and immerse myself in my own world.

2. It may also be that boys think black is cool and show their simple and cold temperament.

3. It may be meaningless.

I hope my answer is helpful to you.