Don't do anything - especially if the answer is short or simple.
You have absolutely no proof the user copied you. So what if they posted after you - that doesn't mean they saw your answer before they posted theirs. They may not even have got the "new answer" notification before they posted.
This has happened to me before several times - I have posted a very similar (almost identical) answer as someone else, and others have done the same to me. It happens, get used to it.
Just to be clear, let me explain how this can happen:
- user scans down the list of questions, using the middle mouse button to click on question titles they want to look at further
- question pages open in a new tab
- user then iterates through these tabs answering questions as they go
- usually a "there are new answers to this question" notification will appear if a new answer has been posted, however
- there is no guarantee this prompt will appear
- there is no guarantee the user will take any notice of it
- even if they see it there is nothing to stop them carrying on with their answer
Because his answer is going on the top of answers' list and is earning reputation
This is exactly the reason why the order of the answers is randomised with the accepted answer shown at the top. If the other answer hasn't been accepted then you will both be at the top of the list at various times.
Flag it for moderator attention? I have done that, but I received this answer: "declined - flags should only be used to make moderators aware of content that requires their intervention."
The moderator did the right thing. If you had posted a complex answer that couldn't reasonably have been accidentally copied then they would have something to go on. For simple short answers you have no grounds for claiming plagiarism.