Sometimes when a horse behaves like that it's not them being bad or stubborn at all. The horse might be terrified, maybe more terrified than the rider. If the horse does not have experience being loaded and trailered and taken on a trail ride it might have been the most frightening thing in his whole life. Some horses have been isolated from other horses and are social misfits. When they go out with a strange herd they not only do not trust the other horses, they sometimes feel they have to defend themselves (by kicking out). I bought a horse that had very limited social interaction with other horses and she kicked out when others came near her for the first year. She was frightened when other horses got too close to her. She would have been happy to run away from the strangers if I had let her.
Some horses seem to view being with strange horses the way you would view being dropped off in a dark alley with four or five strangers. Are you going to feel safe just because they're people?
I would say that a horse that rears when being loaded in a trailer could be asserting its will. OR, more likely, it hasn't been trained to load at all and is frightened to be forced into a small, dark box. Trailering is not natural for a horse at all.
An insecure horse needs a very confident rider that offers security with all of their body language.
I'm not saying this is definitely the case, but I think people often blame horses instead of seeing things from their perspective.
This horse is probably wrong for his owner though, unless the owner really wants to slowly bring the horse along and start by riding in situations where they are both comfortable. It sounds like too much, too soon to me.