I want to know a lot more about the situation before I make a recommendation.
Is this horse boarded in a stall?
Is this horse turned out any?
Is he receiving a lot of grain? He shouldn't be.
If no one is working this horse, why is he not just turned out?
I think you are in way over your head, especially if he is not getting a lot of exercise.
While you may think he is being neglected, the truth is that no handling and being turned out would be much more preferable to poor handling that allows him to get away with things he should not be doing. No handling beats poor handling every time. A big, strong willed horse can get very spoiled very quickly.