I'd guess the horse is probably being hurt, despite your good intentions. Check out tack, back, other potential issues... Balance International have a lot of great info on their site regarding *comfortable* saddle fit.
Assuming there is no physical prob, or you've rectified it, I would use approach & retreat & positive reinforcement, to teach him to *happily* accept the saddling process.
*But* I would not be uncomfortable with *instant* punishment/negative reinforcement either. While I'd aim to train in such a way that it was likely not necessary, if I thought that biting was going to be likely, I'd strap a brush, rough mat or something spikey around the likliest part of my anatomy, so that I could go about my business & if he bit me, he'd cause discomfort *to himself* as opposed to me punishing him directly & after the event.
If he's fine being saddled, until it's time to do up the girth, you might start the 'retraining' there, or go back as far as necessary, if he's not fully comfortable & confident with prior steps.