hmmm...i too have a sensitive horse to clippers.....well did have! now shes super (ish!)
last year i purchased some quieter battery clippers and spent a few weeks desensitising her. Last year i spent ages with a tub of treats and rewarded for everything positive that she did and ignored the bad. It worked really well and i did manage to clip her, but she was really unconfident with them on her neck so her neck was the last bit i did and i did a tiny tiny thin line up to her chin and that was it.
This winter all my hard work from last winter actually paid off, im not sure if you have seen my other thread (
http://www.newrider.com/forum/showthread.php?t=216516) i did it over three days only this year, i rode her out looking silly but to be honest i didnt want to push her confidence limits! i completed the clip on monday and she really really tried to be good (was still a bit unconfident but didnt do anything bad at all!)
have you tried clipping him on his belly/shoulder? Moet is totally relaxed on her belly and shoulder but gets more nervous on her neck. i start with her shoulder and belly then move up her neck (although did actually do it backwards this time! lol!)
Moet is fine with them touching her but does sometimes does get scared when it cuts the hair, it must tickle. What i found helped this year was cutting a tiny bit, stopping and rubbing it vigorously with my other hand, then repeat, after 3 times she didnt react.
if you desperatly want your horse tidy for the show i would be try clipping after giving sedalin, if no success then get the vet out to sedate. If not too bothered then take as long as it takes to get him used to them.