A boy chatted with me and said he was Neptune. What does he mean, is it really Neptune?

It's just a joke, why take it seriously! In fact, if you have seen the movie Aquaman, you will know what the representative of Aquaman means. He is a symbol of courage, strength, power and justice, and an indisputable hero. When a boy says he is Neptune, he should want you to cherish him, or ask him for help when you are in trouble.