Horses are constantly communicating with us and they do this most powerfully when they do something we consider to be a vice.
So, if your horse bites you, the first thing you must ask yourself is WHY? What does this behaviour indicate? Obviously it means he doesn't want you near him, but why? It can only be because he associates your presence with pain.
There can be many reason for this. It may be that you have hurt him physically or frightened him, or that someone else has hurt him or frightened him in the past. But there's something else that could cause him pain. It could be that the saddle is hurting him. Because you are the one that puts that saddle on, you are the one that causes him pain and you are the one he wishes to keep away from him by biting. Many people will say that they have had the saddle fitter out to fit their saddle and because of this they feel that they have done all that needs to be done. But, the saddle fitter can only be sure that the saddle fitted on the day that they fitted it, because both the saddle and the horse can change shape. A fortnight later both could have changed shape, particularly if the saddle is newly stuffed and/or the horse has had a change in diet or exercise.
Use The Perfect Fit Saddle Fitting Kit to check if the saddle fits. If it doesn't, be sure you get one that does and don't ride him in the old saddle until you do. Using the kit, check the saddles's fit at least every month, or more often if there are changes to your horse's diet or exercise.
When you are certain that the saddle fits well, it could be that your horse remembers the pain he was in previously, or, he could still in pain from the damage done by that saddle. If this is the case, you must have his back checked by an equine physiotherapist, chiropractor or osteopath. In fact, if a horse exhibits any unpleasant behaviour, the first thing to do is have him checked out by one of these professionals.
Sometimes it can take a while for a horse to trust that the saddle isn't hurting him, so be patient with him if the biting doesn't stop immediately.