What does it mean to reward a boy for sending me a text message: "Well, Lu"?

I suggest you give him a "Lu" too, and then he will say what his Lu means. If he plays pranks and curses, it's like dealing with a man as he deals with you. If he means like it, you can say that your Lu is leaving and keeping him away. Of course, you can also have other explanations and other answers. It is also possible that he sent something strange without explanation. Never mind, you can send it back as usual, as long as you explain it clearly.