You could use a twitch if he's very bad about it :)
** before starting, teach the horse to lower its head by pulling down on the halter.
With the clippers turned off, move them around her face.
Still with the clippers turned off, touch her with the clippers all over. Retreat every time the horse relaxes.
Tip the horses nose toward you and turn on the clippers. Move the clippers all around without touching her. When she relaxes reward by turning the clippers off. Repeat.
Now touch the horse with the clippers on starting at the withers and working your way up the neck to the face. Reward by going back to the withers.
** once you get through all the steps, put the clippers away, do some other groundwork, and come back to it in a little bit.
This should take time. It's a process. Don't rush it and eventually he will get used to the clippers.