My QH was terrified of the clippers. What I did was buy a cheap pair of those hand held, cordless clippers (by Wahl, I think). I worked with him daily until he got used to them. I didn't use treats, but rather used a round pen. That way if he broke away from me he couldn't get far or do any damage. Also, sometimes we would take a break from the clippers and do a little work to keep his mind in the right place.
If you don't show, the only thing I'd be clipping is his bridle path. The hair on the ears and the whiskers on the muzzle and around the eyes are there for a reason.
Until your horse gets used to the clippers, use a pair of scissors to trim the bridle path. Just be careful in case your horse is afraid of the snipping noise, too! (mine was