I am NOT a fan of Clinton Anderson. But. This works. You want the horse to WANT to go in the trailer, you want to fight them to keep them out. The way you do that is you want them to associate the trailer with a place of rest and doing nothing but relaxing (Horses LOVE doing nothing) and you want outside the trailer to be the place to work her feet (Or his).
What's not shown in this video is that he will get a horse to lunge, always coming around to face the open part of the trailer. If the horse slows down and thinks about going in the trailer, he lets them stop, then tries to load them, and if they don't want to, he works them again... rinse and repeat. The horse will get closer and closer to loading as they come to associate the open trailer with doing nothing.
This is my husband's mare. In October, 2018, she got in the trailer and while I was tying her head, she decided to lose her mind, sat back, and nearly popped two of my fingers off - they were still tying the slip knot. After several months of a constant fight to load in the slant, I asked my cousin's daughter, who had just gone to a CA clinic on trailer loading, if she could see what could be done with Gina. This is the end result. I didn't get video of the very end result - you can open the trailer, point her that way, throw the lead rope on the neck and she'll load without any problem at all.
Had the same problem with Sarge. He didn't want to load in the slant, he wanted to just walk off and drag J along with him. Video 1 is the beginning, Video 2 is the end result 20 minutes later.
Others here may have better suggestions, but this is what has worked for us.
ETA. I agree with AnitaAnne, that trailer looks to be a squeeze for him and he may be very uncomfortable, mentally and physically in it. Rule this out FIRST before you try any of what I posted Up There.