Assuming the horse is otherwise well behaved and goes where you ask it and obeys commands in every other way.
Have you checked that the floor of the trailer is safe - horses can sense things that we don't always see - if you have rubber matting in it then lift it up and inspect the floor underneath and the chassis
Remove the partition so the horse has more room and doesn't feel so enclosed
Open all the windows and the door at the front if it has one for same reason
If your trailer is a walk through front unload can you have another horse that loads well walk through in front of this horse and just follow behind him through a few times?
Be patient, don't try to rush things or lose your cool. Use pressure and release at the right times
Don't encourage the horse to go on by holding a bucket of food in front of it - they will most always grab the food and then run back
You can reward with a treat when the horse has done what you asked of him
Don't stand in front of the horse and try to drag him on - it rarely works
Wear gloves - rope burns are painful
A horse like this sometimes needs a strong experienced handler and a quick thinking assistant or two that's used to dealing with this habit to stop them running back. Its too easy to get injured in these situations and it might be the right time to call for help