How many days until your show. I havent had experince with this but I heard a way to get a short coat without clipping is to stall the horse with lights on it and a stable sheet. IMO I would keep the sheet off, the lights on him and a fan to keep him cool because it's been super hot down here but I don't know about where you live..
Thunder 9yr. black Mustang Appy/Paso
George 18yr. Red roan QH
'Grab life by the reins'