You guys are getting hung up on the antlers when it's not an issue. You should have worded the question without mentioning the antlers, some people here are antler crazy.

He has tags for both, he doesn't care if it has antlers or how big they are, so why should it matter if he can identify them?

Now if it's too dark to tell if it's a deer or not, that is a different story. It would suck if you accidently shot someones pony or mule that was loose. I have been in the situation you have been in though. You can for sure make out that it is a deer, but because of the fading light and the background it is hard to make out the rack.

I guess it's up to you in the end. Let us know what happens if/when you kill it.

Paul