Try this. Go back and desensitize him. Take a cotton lead and put it around his back and tighten it slowly and lightly. If he does try to bite then it must be the cinch that bothers him. Rub him as much as you can on the belly and around the girth area. Keep doing it until he relaxes. The same goes with the lead. Tighten, loosen, tighten, loosen. Only loosen when he isn't reacting.
Check his back by pressing from the withers to his butt. Gently but firm. If he reacts then its his back. Just test him out. Ulcers? If all else fails, call a vet for a check up. Good luck Posted via Mobile Device