If indeed the horses are in that bad of shape and you have photos to confirm it, then its time you went one step further. If AC will not come out or comes out and then gives warnings, then go over their heads. Either contact the head of AC or go to the state level and start showing photos... Get in contact with a TV station and tell them the story, that the man is not feeding his horses, the water is gross, tell about one dying, and that AC comes out and gives warnings but then does not recheck. IF your story is as bad as you say and IF the proof shows it, then contact AC one more time and tell them that you are going above them to get somethinng done.
By feeding them, you are doing what everyone is saying, you are enabling him to continue treating the horses bad. He could actually sue you and your friends for coming onto his property, even if you are throwing hay over the fence and could very well accuse you of hurting his animals. The key point here is HIS ANIMALS.
I hate when animals are starved, etc but until the law is on your side, YOU are the one who will end up getting in trouble for butting in when it doesn't concern you.
You can persue this, just make sure that everything is just like you say, not just what you think you are seeing
Also as VelvetsAB says, how do you know he is an alcoholic?