You and your co-worker agree on the one critical fact in this situation: you presented your ideas to him, and he presented them as his own at the meeting. Maybe he previously had similar ideas, or maybe he did not. Perhaps he had something similar to them, but could not put them into words. None of that matters. You articulated them first, and they are yours.
Besides, if he actually had ideas that were similar to yours, don't you think he would have said something when you met with him? It is sheer folly for him to let a few days go by and then blithely contend that the ideas are his.
Unfortunately, you are not likely to change your co-workers' mind, and if you tell your manager that those ideas were yours, he or she is not going to know whom to believe, and that could lead to a bad outcome for you. The good news is that since you are capable of generating creative ideas, you are likely to do so again. And you are not likely to have them stolen again.
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