|It sounds like you believe that if you say something, you will look like a fool, and if you say nothing, you will feel like one. The fact is that it would be foolish to do nothing in this situation.|
There are basically two explanations for your manager's behavior, and neither is particularly uplifting. The first is that he is the kind of manager who goes through the charade of listening, while hearing very little. In this case, he was not paying attention to you and has no recollection that this idea was originally yours.
The second explanation is that he listened carefully to what you said, and then took the idea as his own. When it comes to ideas, a manager who does this has no idea as to what management is all about.
Either way, it is time to meet with him. You should use a candid and businesslike approach, based heavily on asking questions. Review the facts regarding the original conversations with him in which you presented your idea, and then indicate that you are puzzled over what happened from that point.
Whether the light goes on in his head or not, this entire situation sheds light on the need for you to use more assertiveness, repetition, and documentation on key issues or suggestions that you discuss with him in the future.