I'm with Joe A. Wait it out. A head-on deer close enough for a shot is not a static situation. It's going to change position sooner or later and unless it happens too fast, it's going to change into an acceptable shot.
I shot a doe that was walking right at me once and she stopped just about 10 feet away and stared at me and then barked a couple of times and ran off but only a few feet and stopped broadside. As she turned to go I drew and when she stopped ... thwack. Sure, she could've kept going but didn't.