Some horses will happily walk into a trailer the first time you ask them to but most of them won’t so don’t expect him to do so straightaway and be patient, after all, how would you feel if somebody forced you into a dark room and you had no idea what was in the room? Instead, it’s important to take your time, have small goals and reward your horse every time he does what you ask of him. Never tell him off for doing it wrong, this will only reinforce his fear. It’s important to not rush your horse, take your time and do ten minutes each day, it might seem like it’s taking forever but in the long run, it’s far better for successful loading.
Don’t try and load your horse immediately, he needs to be comfortable around the trailer to start with, instead walk him past the trailer so that he doesn’t automatically think about it as something to dread. Do this for a few days then when he’s happy, walk him to the side of the trailer and then give him a treat. This will help him to realize that a trailer isn’t anything to be scared of. Lots of patience and slow breathing is always good.