No fingerposing, the left arm looks reaally weird, also, I think the dude should look not up, but to our left, since that’s the place where zombie will come.
Good concept though there are some problems composition wise.
To start, there’s a lot of empty space, as beautiful as the roof of that building must be it throws the picture off. Also the focul point seems to be the radio set while both subjects zombie and survivor are blurred.
Posing wise Jericho has it, sit down and pretend you have an imaginary shot gun if you really need to get a natural pose.