The police won't release the autopsy report for the same reason they won't release the video. It most likely does not support their position.
There is a difference between facing away, and having your back to someone, and fleeing, where you're actively running away. My guess is fleeing is the last thing they wanted to do. Nor did they want to confront the guy by approaching him. I'm guessing it happened exactly as one would think it happened. They had their backs to the shooter, and were trying to move their son away from the situation.
Again, I think it's clear that the shooters position would carry a great deal more weight if it was shown he was still being attacked by an approaching soon to be victim. My guess is the autopsy report & video put that in extreme doubt.
He's a cop, therefore he's above the law. When you're special, there are a lot of things you can get away with. That's the way things work nowadays. And when they want to keep the public opinion out of it, they just cover up whatever evidence might contradict their position.