More likely he is excited to be going somewhere, not nervous. However, the symptoms may be similar, and quiet is what you want. Take him for short rides to nowhere in particular (around the block, so to speak) and then return home without unloading. Eventually, he won’t know if he is going somewhere exciting or just for a boring ride (you will obviously still have to deal with the extra things done for showing as elements of excitement). And, of course, you will need to make sure the trips to somewhere are fun for him, not just fun for you. Most racehorses enjoy racing, and are usually very good shippers – often better than horses who haven’t traveled a lot.
It is never the horse's fault