You need to take a moment, sit down and think about this seriously. You know this guy better than any of us could - is he a violent person, or was this some horrible bad day resulting in a horrible mistake? Was the man remorseful of his actions, or did he simply not care? You need to sit down, and think - do you think this person will hurt this dog again if it continues living with him?
If the answer is 'Yes', you have two options: First and foremost, I'd offer to buy the dog off him. If he's hurting the animal, he won't put it before personal gain - people like that don't. It's also the 'easiest' option. If you can't afford the dog due to finances, space, or don't want a dog, scout around for a friend who might and explain the circumstances. I gaurantee you'll know someone who will.
Secondly, you have the option to report him to the police or animal welfare services. You cannot leave this animal in the care of an abusive owner; it hasn't been with him long. It's showing signs that it doesn't know what's going on - it isn't learning that this life of fear is 'the way' of things yet. It can still be saved from the cruel fate of constant fear and aggression that'll only result in it being put down.
You seem like a decent person, but you have a very, very important decision to make, and the ability to do something this creature cannot possibly do; save it from a life of horror if that's what you deem to be a possibility for it.
Again, you know this man - If there's even the slightest chance he's hurting the dog, you have to remove it from his care. Don't hit him, don't post shit online, all that's irrelevant and doing nothing to remove the dog from his care. Focus on that first and foremost. You have two days to decide this dog's future. Please, use that time carefully.
