Ha! Been there! LOL No two horses are identical, but here’s what worked for me.
I found that doing a lot of backing-up exercises on the ground helped, and so did clicker training. It helps a lot if he’s already familiar with the clicker-training concept so you can cut to the chase and just use that. I added a cue word and once he was solid with that we began working slowly on the trailer. Don’t put him all the way on at first. Give him an “on” cue and an “off” cue that are different. Cue him “on” and work him till he will put just his front feet on, then cue him “off” and let him back off. Click-treat on both. Continue that on the ground for a few efforts, then back to the trainer. More on, and back off. Even further on, and back off. Keep it up until going in and backing off are making him happy. This could take a while. I don’t recommend using a butt rope or one in front of the chest. Most horses tend to move into pressure, so his first reaction is going to be the opposite of what you want. I had much better results tickling his lower legs with a dressage whip until he was annoyed enough to move the leg I was tapping.
Good luck! I know how scary and frustrating this problem can be. We actually opted not to take one horse off the trailer at a show because we were afraid he wouldn’t get off and, if he did, that he wouldn’t get back on.
Horses In the Yard