He does this as you're bringing the chinch to cinch it up? If so, he's just cinchy, nothing is "wrong" with him health wise. At some point in time others who cinched him up either did it waaay to tight, or didn't attach it properly and it hurt him. It is now a defensive move for him. Just take your time, don't put up with him biting you. He may never get away from this, unfortunatly. But with some help he may make some great improvement.
Make sure you don't overtighten the cinch. When you first tack up just tighten the cinch just enough so the saddle won't fall off, then once you get everything else ready to go, tighten it a bit more. Then walk to where you are going to get on, and do one last tightening on it. Makes it a lil easier on the boy.
~The Almighty put hoofs on wind, and a bridle on lightning, He then called it horse.~