(click on the gif ( by Gerda Dunkel ) to enlarge it)
No. I don't think so. If he were shot in the back of his head, we'd see it just as we see the shot to the side of his head. That little head-nod prior to the fatal head shot was probably just a little head-nod. The head naturally does that. If you stop holding it up, that's what it does. He was tired. Stop reading so much into it. It's nothing.