Hi Rebecca! You should keep him occupied with a hay net, something to play with, calming music and sometimes he needs calmers.
Sometimes horses arre nervous because they fell trapped so maybe get him/her used to the trailer a few days before the competition. Maybe trailer him like if he was going to a competition or something but just do a tour of the barn and see how he reacts then.
A hay net would keep him occupied!
Some horses are different and you can use a Haynets, calmers and some horses even respond to classical music which is definitely worth a try because my horse responds a lot better with it playin
You can give him a calmer booster called magic
Bach flowers are really good! I've also heard about a product than you can get with your vet, but I don't remember the name, you should ask him anyway! But as Adeline said, the hay net can be a good point to begin with :)
Put hay net, he'll be concentrated on eating and will calm down .. If he's really too dangerous you should give him plants products to help him to fight against anxiety :) You also can see if he's calmer if there's an other horse during the travel, if he prefers a truck or a trailer, for exemple some horses don't bear to be in the opposite direction of the road :)