Does he let you comb his mane? Many horses have had their manes pulled too aggressively and they learn the shaking trick. With patience, I have found that often I can overcome the resistance by lots of petting and desensitizing. When I can comb the mane easily with no reaction, then I will pull no more than 3 or 4 hairs. Then I stop and go back to brushing and petting. It helps to do this after you work the horse and his body is warm, the hairs pull out more easily--and he'll be more relaxed. There is an old trick to shortening the mane that works great, it's fast and you don't get the scissor cut look. Take a clipper blade---the top blade from a Clipmaster is what I use (I think it's the top, it's the longer of the 2 blades.) Just shear down through the mane at about 4" to 6" from the neck like you're combing it with the blade. Work up and down the mane. You won't be thinning it but it can look OK if the mane is not super thick. I do this to shorten the long manes and finish with pulling them. To make sure your horse stays OK with mane pulling, never pull more than a few hairs with each tug and work up and down the length of the neck instead of staying in one area. And, if the horse is just being a butthead, I have resorted to giving him a smack with the flat edge of a plastic sweatscraper, but that was before I was enlightened by all of the politically correct people that now run our lives.