I have bought a 2 1/2 yr old who is VERY head shy! It has taken me 2 months of everyday practice to now have him dip his head for a halter like a pro. But now the bit… when I hold the bridle in front of him for the normal way of putting a bit in the mouth, he freeks. The only way I have been able to get a bit in his mouth without fighting him or having him jerk his head up, is to hold the bit in both hands, use two fingers to open his mouth and insert the bit, then bring the bridle up over his head and unbuckle the halter and slip it beneath the bridle.
The first thing to remember is that you must be extremely careful and gentle when bridling any horse. If the bit clanks against the teeth while bridling, this can give any horse a good enough reason to fear being bridled. Always look where you are holding the bit and once the bit is in the horse’s mouth, be sure not to let it fall and clank on his teeth while you are bringing the bridle over the ears.
It will take a lot of patience on your part to get your horse over his bridling fears. If he is head shy, you need to work with him extensively at sacking out his head and face with everything from your hand to plastic bags, lead ropes, clippers, bridles, halters. And whatever else you may find laying around the barn that will not hurt him.
Until the horse has completely overcome his head shyness, he is not safe to ride. A head shy horse can get you into a world of trouble when riding.
Also get the horse used to having your fingers in his mouth. Of course, you should be careful where you put your fingers. Start by inserting your finger in the bar of his mouth, where there aren’t any teeth. Rub and massage his gums. You can even rub your finger in-between his upper lip and gums. Most horses eventually find this very pleasurable. Do this until he is comfortable with having his mouth handled, and he will be much easier to bridle.
Once you have completely sacked him out with many objects (including a bridle) and he no longer fears being touched on his head, he will accept the bridle with no problem.
For more information on sacking out and headshy horses, you can read the following two articles, which address your situation: