Dreams about locks and luggage often symbolize security and protection. In your dream, telling your son to buy a lock for his luggage may represent your desire to ensure his safety and well-being. Luggage itself can symbolize the burdens or responsibilities we carry, and by advising him to secure it, you might be expressing a concern about what he might be taking on in life.

The suggestion to use a familiar number for the lock could indicate the importance of relying on past experiences or established patterns in his life. It might reflect a sense of comfort in familiarity and an encouragement to draw upon what he already knows to navigate challenges.

Overall, this dream could be highlighting your protective instincts as a parent and your hopes for your son to be cautious and prepared as he faces the world. It may be beneficial to consider if there are any current situations in your life where your son might need extra guidance or support.