REPLY: Hi. Thanks for writing. His head flinging there is maybe about pain/discomfort, or perhaps (probably more so!) just total cluelessness about what you want there. Back up to ground work as the in-saddle asking for back up is too "A to N" a leap for him in particular there (leaving out the necessary A-B-C learning steps in between for him to better understand).

First, in the ground work, work in natural horsemanship halter with 12' lead tied on (like you can get here if you don't have one): CLICK HERE

And teach him this ground leading/back-up exercise for a start (it'll go a long way multi-directions, you'll see, including teaching him better to yield to pressure in general, but also will well teach him how to back up on cue from the ground for a better start):

Leading Problems - Horse runs over human/How to teach horse back-up cue on ground

Next...once he's got that above exercise down really well, then use the bridle/bit and teach him how to back up now with that, like you will be doing in saddle, which I'm going to go into next here, but work from the ground first (though you can teach this next step from in saddle if you want; I just like to get as much teaching done from the ground as I can first, depending on the individual horse).  As for the bit to use: I only use a full cheek snaffle bit for training/riding, like you see here:

Bit Problem -  Having trouble controlling horse, what bit to use?

There are a number of ways to teach the back up in saddle and let me show you a couple here and you can choose what works best for your horse there.

1) One good way to teach the back up for riding, for a start with many horses, is to use a half-halt gesture that really encourages the back up easily in so many horses. Here's how to do that:

Collect the reins in each hand, taking the slack out of the reins (you're going to do this on the ground first, standing next to the horse's neck, reaching over for the outside rein -- if you're too short & your horse too tall, you can actually teach this still from the ground, but with your arm under the neck holding the outside rein). Do not pull the reins after you've taken the slack out, just take the slack out to the point where the bit is almost making contact with the corner of the mouth for a start. Hold one rein in each hand with your pinky fingers down, thumbs on top. The reins are making contact with your pinky fingers. Now, with your pinky fingers, make a silver dollar-sized circle in place there. The silver dollar-sized circular motion should be in the direction of toward the horse's head, then up and toward yourself & down around again. The circle you are making there is like a wheel rolling toward you. Make sure this circle you are doing with your pinky is no bigger than a silver dollar. Circle from below to above and back in order to oppose the natural forward motion of the horse.

Repeat the circling motion continuously until the horse begins to move backwards, then release all pressure instantly. The trick here is not to "pull" on the reins or bit with this method, but rather this silver dollar-sized circular motion "tickles" the bit in the horse's mouth. This rhythmic tickling of the bit really encourages a horse to step backwards with very minimal pressure, quite naturally. Real important to release all pressure (shoot your hands forward and put slack in the reins instantly) for each positive try on the horse's part to step backwards, and this first attempt on the horse's part may even entail the horse just starting to lean his weight backwards, like he's preparing to step back. Release quickly (slack in the reins) for the positive "try" right answer. Rub the neck (slack reins still in your hands) for reward after every positive step backwards, with a heartfelt "good boy!" and he'll get there quicker.

Try that and keep soft hands there, asking for just one step at a time for a start. Quick releases for right answers and lots of hearty neck rubbing rewards, and the horse quickly learns how to back up there. Give that a try and you can build from there, asking for more and more steps until the tiniest of pressure back will have your horse backing up nicely, and then do this from in saddle. Collection can come later, but this is a very good baby-stepping way to teach a horse to back up in saddle, without a fight, and with the horse softly complying most every time. Be patient, be consistent, reward to let the horse know he did the right thing there, and you'll get there.
 

2) I wanted to add in here a second approach because sometimes what works with some horses doesn't work with another, hence it's helpful to have a few "tools" in your "training tool bag." While I think the above #1 approach will probably work best with your particular horse, to minimize his tendency to fight the back up cue there, the following is another that I find works fine with those horses who don't fight there and who learn very quickly as a rule. Here's how to do method #2 -- again you can work from the ground first if you want, or this can indeed be taught from saddle with the easier horses -- I just tend to work from the ground first to introduce new things, then repeat it in saddle:

Collect the reins in each hand, taking the slack out of the reins (you're going to do this on the ground first, standing next to the horse's neck, reaching over for the outside rein). Take the slack out to the point where the bit is almost making contact with the corner of the mouth for a start.

Slowly pull the reins downward & back evenly with slight pressure & then hold that pressure steady -- what we're asking for here is for the horse to bend at the poll, which means: tip the nose down first, arching his neck ("collecting"). Release instantly when the horse gets it right or even semi-right (the "try). Do multiple repetitions of that until the horse bends at the poll more easily. The horse may take a step back as he tries to find the right answer there and you can choose to release for that if you wish, but I like to work for bending at the poll first sometimes so they learn how to collect their body more correctly for the back up, when going this method route. When a horse bends at the poll (the poll is the back of the horse's head), with his head down and tucked in slightly, and then backs up, he's holding his back/frame correctly for the backup. If his head is high when he backs up, that means he's hollowing his back (sinking down his back) for the back up and that is not only more uncomfortable for him to be doing with a rider on his back, but it is not correct "collection" overall and it not as good for the horse "chiropractically," you might say. So, I kind of work on both when going this route -- teach the horse to bend at the poll, then back up, in that order. Though there's nothing wrong with using pressure/release there like I'm showing here to just get the back up, then work on the collection/bending at the poll. Each horse learns differently and you have to do a "feel" read for what works best for the individual horse you're working with at a given time.

Use pressure/release with the two reins -- holding the pressure steady back and releasing the second the horse gives a back-up try, and the horse will start "connecting the dots" that when he feels dual pressure back on both reins, he is to step back and away from that pressure. Remember: all horses learn from the release of pressure what it is you want, not necessarily the pressure itself, so get you release timing very fast for "right answers." It's all about the release you give quickly for right answer increments there. Just one step at a time, releasing for every step. What's a good release? Nano-second fast, putting prominent slack in the rope and then stroking the neck, praising the horse, so he knows he got it right!

All of the this (and much, much more) is taught in my Whispering Way 12-Step Total Training System DVD set. Usually it helps to see this natural horsemanship art taught visually, to really understand how it is all done. You can get that DVD set here: CLICK HERE

And incidentally...my Whispering Way Complete Training Package contains all my videos and training tools that you need to train or retrain your horse yourself the natural horsemanship Whispering Way. You can check out/order the Whispering Way Complete Training Package on my web site here: CLICK HERE

Either method you try, when you do this from up in saddle, you're going to want to add in your body language as well there. When asking for the back up in saddle using #1 or #2 above, shift back in the saddle, shifting your weight back as you ask, and I like to add in a "kiss" sound to let the horse know I want: movement. This shifting your weight back releases your weight off of the horse's front quarters and allows him to back up correctly.

Either way, be patient, break things down into smaller baby steps for the horse, use lots of releases for right answer tries, lots of praise and reward, and the horse will get there.