If the situation is getting really dangerous, you should use sedatives. I know some people are reluctant to have recourse to them but it can be better for everyone. You can also try Bach flowers, it is more natural :)
Putting cotton in the ears is a good idea. You can also try to cover the eyes, some horses are less afraid if they are in the dark, they can relax more easily (but careful, actually it can also panic some horses!). If necessary, you can give tranquilizers. This situation can quickly become dangerous for the horse and for you so be patient.
It must be strange but if your horse is afraid of the shears' noise, you can use an electric toothbrush to desensitize your horse to this noise. You turn on the toothbrush and rub it on the horse's body as if you were shearing him!
I know you can put some cotton in his ears so he won't ear the noise. A lot of horse are afraid of the shears' noise and not of the feeling of being shaven!