** Awnser every single question in this post. It is what we need to know to help you. Because currently, you are telling us close to nothing **
How long have you had him and how much experience do you have with horses?
When you say "he is willing to run" what does that mean? You only run him? Or he is a racehorse, and that is what he does as a career?
What is he being fed, both grain and hay, is he turned out any, and does he have anything, if he is stalled, to alleviate boredom? Can he see or interact with other horses? Is he a gelding? What breed and age?
How old are you, and what experience do you have with horses, and if young, is there someone around with more experience? Where did this horse come from, is he new to you, or one that you have had for a while?
When he bites the stall bars, does he hold onto them for a long time, or seem to just chew on them for a little while?
Does he make a noise when biting his stall bars?
How often does he do this per day?
Take your time to awnser everything for a quick response. If you keep up the choppy awnsers, we will be here for months.