Get busy now, gently massing his ears several times a day and you're most likely still not going to get him de-sensitized by clipping day but he may tolerate the clippers IF you buy the Wahl fit-in-your-palm battery operated clippers. They are only around $20 and will do a terrific job.
I would not tie him, you don't need two hands to massage ears.
Buy one of those dog hair mits at Walmart and start massaging his ears on the outside, slowly working your way to the inside.
Don't make eye contact - I have noticed my ear sensitive horses do a lot better if I reach up to their ears while I am looking down at the floor.
Even my worst "ear horse" will let me clean his ears out as long as we are not making eye contact.
I don't understand, but it works.
Most importantly, you need to be fussing with this horse's ears at least three times a day --- if you're in school or working, get up extra early and get to the barn if the show is that important. There are no magic bullets unless you want to drug the horse and that's not a good idea if they test at the show