Has he ever been in a straight load? Can you take the partition out? Some horses are scared of the tight quarters. For teaching it might be beneficial to take it out until he's loading and unloading.
I've had horses with trailer trauma, and each one of them has something new to teach me about getting them to load.
There's the get their mind working before loading, the rump rope, the grain can, the trail-of-grain, the stand at the back and let them sniff until they get in, and so on and so forth.
I've had to use rope halters on horses who wont budge, so it hits the poll more and gives them a better "go" cue. I don't like tieing them in the trailer in one, however.
Usually, I lead them up to the trailer casually. If they stop, I keep going and wait for them in the back of the trailer. I let them sniff if they need too, then I ask with an even pull to come forward. With each step, they get a release of pressure on their halter and usually a nibble of grain if they continue to be willing. Then I ask for another step, and another. If they take a step backwards I tell them a firm "NO" and cue to go forward. If a horse is really stubborn I've had to use a rump rope (and have had horses lunge forward on me, not my fav) or have someone standing somewhat behind them with a lunge rope so they can smack the ground with it when they step back.
Usually the horse realizes, "Hey, everytime I even lean forward I hear 'good boy' and if I step I get food, when I go back I got crazy back there with the whip." And they give up and come in for me, where they recieve their grain.
If they get a foot in, or both front feet, and they want to back out, don't pull on them as that would usually make a horse rear. Let them out and give them a good boy for just trying.
If he's really had some trauma, then take it step at a time. A little closer each day. Don't expect them to come all the way in on one day, but don't give up when they're refusing either.
Mother to three amazing rescue babies<3 ~Pele ~ 4/5 year old palomino appy filly.
~Papillonn ~ 20 year old arab gelding. ~Ruby ~ 3/4 year old flaxen chestnut BSP filly.