What does it mean when a boy ask a girl that who is your lucky boy?

When a boy asks a girl "who is your lucky boy?" it could mean that he is interested in knowing if she is currently in a relationship or if there is someone special in her life. It could also be a playful or flirtatious way of asking if she has a crush or someone she considers lucky to have in her life. Ultimately, the meaning would depend on the context and tone of the conversation.