"Chill" is a natural way to take the edge off
It is a liquid "oral"
You could ship him with another "buddy" horse or maybe give him lots of food. Also you can load him at home everyday so he gets used to it
Pick out some time when you can maybe lunge the horse on a line or lead rope around the trailer on either side or on the loading side. Make being outside of the trailer work and then feed him inside. It will make being inside the trailer look easy to the horse and a place to get out of work.
Hook the trailer up to the truck, park it in the paddock, and start tosing his hay and grain in there. He will start to associate the trailer with food, and what horse doesn't think food is a positive thing!
Try putting him in there (not on show day) at his own pace and reward him with treats, food, grooming, etc. and repeat a few times. If necessary, have a horse in there with him as well
Hang a hay net as a distraction, use shipping boots or bandages, and bell boots for protection. Try to trailer him with a friend on the trailer. You can also feed him his supper a few times on the trailer so that he becomes more comfortable with it.
How often does he get to be around over horses,he could just need some time to be a horse. Also, make sure his feed isn't high in sugars.
Make sure there is plenty of food in front of him. Also practice putting him in there and just grooming him and loving on him. Make it a happy experience. Then start the truck up and do the same thing. Hope this helps