And while you say horse has to come out?
No, they don't. They can overheat from getting in a panic and die inside that trailer, or lose it and get themselves cast and die. Or decide to fight their way out by jumping forward into nose of trailer.
No way would I ever encourage someone to do this.
Aside from panic issues...horses will stock up, by not being able to move around could end up with colic and since males stretch out to urinate, might end up with bladder/kidney problems too.
May also be that he would unload better if on other side too? And are your roads crowned like here is US, to where we put heavier horse on side to center of road, or do you drive on left side there?
Might also try blindfolding him and backing him up? But again, this is awfully tight spot to fool around in. Can run lead through front legs and down and pull from behind to back, as have done that.
If horse will not even back over a small rise in boards, I would get some serious vet work done, as to what is going on. I too think pain related.