Thank you for the clarification.

You said your boyfriend currently has the key to the padlock - perhaps instead of taking off the necklace or returning it to him which might cause hurt feelings, or feelings of rejection, you should have a conversation with him including a lot of what you have mentioned above. Tell him you would like to keep the key, and as your relationship progresses, you hope to get to the point where it can be presented to him in a more formal way?

However you go about it, just be honest and stress what you want to progress to and not so much what you simply feel is lacking.